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Versions: 00 01 02 rfc2072                                              
PIER Working Group                                          H. Berkowitz
INTERNET DRAFT                                         PSC International
                                                             August 1996

Expires in six months

                        Router Renumbering Guide

Status of this Memo

   This document is an Internet Draft. Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
   and its Working Groups. Note that other groups may also distribute
   working documents as Internet Drafts.

   Internet Drafts are draft documents valid for a maximum of six
   months. Internet Drafts may be updated, replaced, or obsoleted by
   Internet Drafts are draft documents valid for a maximum of six
   months. Internet Drafts may be updated, replaced, or obsoleted by
   other documents at any time. It is not appropriate to use Internet
   Drafts as reference material or to cite them other than as a "working
   draft" or "work in progress".

   Please check the I-D abstract listing contained in each Internet
   Draft directory to learn the current status of this or any other
   Internet Draft.


   IP addresses currently used by organizations are likely to
   undergo changes in the near to moderate term.  Change can become
   necessary for a variety of reasons, including enterprise
   reorganization, physical moves of equipment, new strategic
   relationships, changes in Internet Service Providers (ISP),
   new applications, and the needs of global Internet connectivity.
   Good IP address management may in general simplify continuing
   system administration; a good renumbering plan is also a good
   numbering plan.    Most actions taken to ease future renumbering
   will ease routine network administration.

   Routers are the components that interconnect parts of the IP
   address space identified by unique prefixes.  Obviously, they will
   be impacted by renumbering.  Other interconnection devices, such
   as bridges, layer 2 switches (i.e., specialized bridges), and ATM
   switches may be affected by renumbering.  The interactions of these
   lower-layer interconnection devices with routers must be considered
   as part of a renumbering effort.

   Routers interact with numerous network infrastructure servers,
   including DNS and SNMP.  These interactions, not just the pure
   addressing and routing structure, must be considered as part of
   router renumbering.

draft-ietf-pier-rr-02.txt                                       [Page 1]

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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . . 1
   2.   Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.   Motivations for Renumbering  . . . . . . . . . . . . . . . . 3
   4.   Numbering and Renumbering. . . . . . . . . . . . . . . . . . 7
   5.   Moving toward a Renumbering-Friendly Enterprise. . . . . . . 11
   6.   Potential Pitfalls in Router Renumbering.  .  .  . . . . . . 16
   7.   Tools and Methods for Renumbering  . .  .  . . . . . . . . . 21
   8.   Router Identifiers . . . . . . . . . . . . . . . . . . . . . 25
   9.   Filtering and Access Control . . . . . . . . . . . . . . . . 32
  10.   Interior Routing . . . . . . . . . . . . . . . . . . . . . . 33
  11.   Exterior Routing . . . . . . . . . . . . . . . . . . . . . . 34
  12.   Network Management . . . . . . . . . . . . . . . . . . . . . 35
  13.   IP and Protocol Encapsulation  . . . . . . . . . . . . . . . 38
  14.   Security Considerations. . . . . . . . . . . . . . . . . . . 38
  15.   Planning and Implementing the Renumbering  . . . . . . . . . 40
  16.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 40
  17.   References . . . . . . . . . . . . . . . . . . . . . . . . . 40
  18.   Author's Address . . . . . . . . . . . . . . . . . . . . . . 40

1.  Introduction

   Organizations can decide to renumber part or all of their IP
   address space for a variety of reasons.  Overall motivations for renumbering
   are discussed in [ovrvw].  This document deals with the router-related aspects
   of a renumbering effort, once the decision to renumber has been made.

   A renumbering effort must be well-planned if it is to be successful.
   This document deals with planning and implementation guidelines for
   the interconnection devices of an enterprise. Of these devices,
   routers have the clearest association with the IP numbering plan.

   Planning begins with understanding the problem to be solved.  Such
   understanding includes both the motivation for renumbering and the
   technical issues involved in renumbering.

      1.  Begin with a short and clear statement of the reason to renumber.  Section 3
          of this document discusses common reasons.

      2.  Understand the principles of numbering in the present and
          planned environments.  Section 4 reviews numbering and
          suggests a method for describing the scope of renumbering.

      3.  Before the actual renumbering, it can be useful to evolve
          the current environment and current numbering to a more
          "renumbering-friendly" system.  Section 5 discusses ways to
          introduce renumbering friendliness into current systems.

      4.  Be aware of potential pitfalls.  These are discussed in Section 6.

      5.  Identify potential requirements for tools, discussed in Section 7.

      6.  Evaluate the specific router mechanisms that will be affected
          by renumbering.  See Sections 8 through 13.

      7.  Set up a specific transition plan framework.  Guidelines
          for such planning are in Section 15.

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   When trying to understand the interactions of renumbering on routers,
   remember there different aspects to the problem, depending on the scope of
   the renumbering involved.   Remember that even an enterprise-wide renumbering
   probably will not affect all IP addresses visible within the  enterprise,
   since some addresses (e.g., Internet service providers, external business partners)
   are outside the address space under the control of the  enterprise.

2. Disclaimer

   The main part of this document is intended to be vendor-independent.
   Not all features discussed, of course, have been implemented on all
   routers.    This document should not be used as a general comparison
   of the richness of features of  different implementations.
   References here are only to those features affected by renumbering.
   Some illustrative examples may be used that cite vendor-specific
   characteristics.  These examples do not necessarily reflect the
   current status of products.

3.  Motivations for Renumbering

   Reasons to renumber can be technological, organizational, or both.
   Technological reasons fall into several broad categories discussed
   below.  Just as there can be both technological and organizational
   motivations for renumbering [ovrvw], there can be multiple technological

   There may not be a clear line between organizational and technical
   reasons for renumbering.  While networks have a charm and beauty all their own,
   the organizational reasons should be defined first in order to justify the budget
   for the technical renumbering.  There also may be pure technnical reasons to
   renumber, such as changes in technology (e.g., from bridging to routing.

   While this document is titled "Router Renumbering Guide," it  recognizes that
   renumbering may be required due to the initial  installation of
   routers in a bridged legacy network. Organizations may have had an adequate
   bridging solution that did not scale with  growth.  Some organizations
   could not able to move to routers until router forwarding performance improved
   [Carpenter] to  be comparable to bridges.

   Other considerations include compliance with routing outside the organization.
   Routing issues here are primarily those of the global Internet, but may also
   involve bilateral private links to other enterprises.

   Certain new transmission technologies have tended to redefine the
   basic notion of an IP subnet.  The numbering plan needs to work with
   these new ideas.  Legacy bridged networks and leading-edge workgroup
   switched networks may very well need changes in the subnetting structure.
   Renumbering needs may also develop with the introduction of  new WAN
   technologies, especially nonbroadcast  multiaccess (NBMA) services such as frame
   relay.  Other  WAN technologies, dialup media using modems or ISDN, also may have
   new routing and numbering requirements.  Switched virtual circuit services such as
   ATM, X.25,  or switched frame relay also interact with routing and addressing.

3.1  Internet Global Routing

   Many discussions of renumbering emphasize interactions among organizations'
   numbering plans and those of the global Internet [RFC1900].  There can be equally
   strong motivations for renumbering in organizations that never connect to the

draft-ietf-pier-rr-02.txt                                       [Page 3]

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   global Internet.

   According to RFC1900, "Unless and until viable alternatives are developed, extended
   deployment of Classless Inter-Domain Routing (CIDR) is vital to keep the Internet
   routing system alive and to maintain continuous uninterrupted growth of the
   Internet....To contain the growth of routing information,
   whenever such an  organization changes to a new service provider, the
   organization's addresses will have to change.

   Occasionally, service providers themselves may have to change to a new and
   larger  block of address space. In either of these cases, to contain the growth
   of routing information, the organizations concerned would need to renumber.... If
   the organization does not renumber, then some of the potential consequences may
   include (a) limited (less than Internet-wide) IP connectivity, or (b) extra cost
   to offset the overhead associated with the organization's routing information
   that Internet Service Providers have to maintain, or both."

3.2  Bridge Limitations; Internal Use of LAN Switching

   Introducing workgroup switches may introduce subtle renumbering needs.
   Fundamentally, workgroup switches are specialized, high-performance bridges,
   which make their main forwarding decisions  based on Layer 2 (MAC) address
   information.   Even so, they rarely are independent of Layer 3 (IP) address
   structure.  Pure Layer 2  switching has a "flat" address space that will need
   to be renumbered into a hierarchical, subnetted space consistent with
   routing. Traditional bridged networks share many of the problems of
   workgroup switches,  but have additional performance problems
   when bridged connectivity extends across slow WAN links.

   Introducting single switches or stacks of switches may not have
   significant impact on addressing, as long as it is remembered that
   each system of switches is a single broadcast domain.  Each
   broadcast domain should map to a single IP subnet.

   Virtual LANs (VLAN) further extend the complexity of the role of workgroup
   switches.  It is generally true that moving an end station from one switch port
   to another within the same "color" VLAN will not cause major changes in
   addressing. Many discussions of this technology do not make it clear that
   moving the same end station between different colors will move the
   end station into another IP subnet, requiring a significant address change.

   Switches are commonly managed by SNMP applications.  These network management
   applications communicate with managed devices using IP.  Even if the switch does
   not do IP forwarding, it will  itself need IP addresses if it is to be managed.
   Also, if the clients and servers in the workgroup are managed by SNMP, they
   will need IP addresses.  The workgroup, therefore, will need to appear as one or
   more IP subnets.

   Increasingly, internetworking products are not purely Layer 2 or Layer 3 devices.
   A workgroup switch product often includes a router function, so the numbering plan
   must support both flat Layer 2 and hierarchical Layer 3 addresses.

3.3  Internal Use of NBMA Cloud Services

   "Cloud" services such as frame relay often are more economical than  traditional
   services.  At first glance, when converting existing enterprise networks to NBMA,
   it might appear that the existing subnet structure should be preserved, but this is
   often not the case.

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   Many organizations  often  began by treating the "cloud" as a single
   subnet, but experience has shown it is often better to treat the
   individual virtual circuits as separate subnets.  When the individual
   point-to-point VCs become separate subnets, efficient address
   utilization requires the use of /30 prefixes for these subnets.  This
   typically means the addressing and routing plan must support multiple
   prefix lengths, establishing one or more prefix lengths for LAN media
   with more than two hosts, and subdividing one or more of these
   shorter prefixes into longer /30 prefixes that minimize address loss.

   There are alternative ways to configure routing over NBMA, using
   special mechanisms to exploit or simulate point-to-multipoint VCs.
   These often have a significant performance impact on the router, and
   may be less reliable because a single point of failure is created.
   Mechanics of these alternatives are discussed later in this section,
   but the motivations for such alternatives tend to include:

      1.  A desire not to use VLSM.  This is often founded in fear
          rather than technology.
      2.  Router implementation issues that limit the number of subnets
          or interfaces a given router can support.
      3.  An inherently point-to-multipoint application (e.g., remote
          hosts to a data center).  In such cases, some of the
          limitations are due to the dynamic routing protocol in use.
          In such "star" applications, static routing may actually be
          preferable from performance and flexibility standpoints,
          since it does not produce routing traffic and is unaffected
          by split horizon.

   To understand how use of NBMA services affects the addressing
   structure and routers, it is worth reviewing what would appear to be
   very basic concepts of IP subnets.  The traditional view is that a
   single subnet is associated with a single physical medium.  All hosts
   physically connected to this medium are assumed to be able to reach
   all other hosts on the same medium, using data link level services.
   These services are medium specific:  hosts connected to a LAN medium
   can broadcast to one another, while hosts connected to a point-to-
   point line simply need to transmit to the other end.

   When one host desires to transmit to another, it first determines
   if the destination is local or remote.  A local destination is on
   the same subnet and assumed to be reachable through data link
   services.  A remote destination is on a different subnet, and it
   is assumed that router intervention is needed to reach it.

   The first NBMA problem comes up when a single subnet is implemented
   over an NBMA service.  Frame Relay provides single virtual circuits
   between hosts that have connectivity.  It is quite common to design
   Frame Relay services as partial meshes, where not all hosts have
   VCs to all others.  When the set of hosts in a partial mesh is in
   a single IP subnet, partial mesh violates the local model of full
   connectivity.  Even when there is full meshing, a pessimistic but
   reasonable operational model must consider that individual VCs do
   fail, and full connectivity may be lost transiently.

draft-ietf-pier-rr-02.txt                                      [Page 5]

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   There are several ways to deal with this violation, each with their
   own limitations.  If a specific "central" host has connectivity to
   N all other hosts, that central host can replicate all frames it
   receives from one host onto outgoing VCs connecting it with the (N-1)
   other hosts in the subnet.  Such replication usually causes an
   appreciable CPU load in the replicating router.   The replicating
   router also is a single point of failure for the subnet.  This method
   does not scale well when extended to fuller meshes within the subnet.

   In a routing protocol, such as OSPF, that has a concept of designated
   routers, explicit configuration usually is needed.  Other problems in
   using a meshed subnet is that all VCs may not have the same
   performance, but the router cannot prefer individual paths within the

   One of the simplest methods is not to attempt to emulate a broadcast
   medium, but simply to treat each VC as a separate subnet.  This will
   cause a need for renumbering.  Efficient use of the address space
   dictates a /30 prefix be used for the per-VC subnets.  Such a prefix
   often needs VLSM support in the routers.

3.4  Expansion of Dialup Services

   Dialup services, especially public Internet access providers, are
   undergoing explosive growth.   This success represents a particular
   drain on the available address space, especially with a commonly
   used practice of assigning unique addresses to each customer.

   In this practice, individual users announce their address to the
   access server using PPP's IP configuration option [RFC1332].  The
   server may validate the proposed address against some user
   identifier, or simply make the address active in a subnet to which
   the access server (or set of bridged access servers) belongs.

   These access server functions may be part of the software of a
   "router" and thus are within the scope of this Guide.

   The preferred technique [Hubbard] is to allocate dynamic addresses
   to the user from a pool of addresses available to the access
   server.   Various mechanisms are used actually to do this assignment,
   and are discussed in Section 5.5.

3.5  Internal Use of Switched Virtual Circuit Services

   Services such as ATM virtual circuits, switched frame relay, etc.,
   present challenges not considered in the original IP design.  The
   basic IP decision in forwarding a packet is whether the destination
   is local or remote, in relation to the source host's subnet.  Address
   resolution mechanisms are used to find the medium address of the
   destination in the case of local destinations, or to find the medium
   address of the router in the case of remote routers.

   In these new services, there are cases where it is far more effective
   to "cut-through" a new virtual circuit to the destination.  If the
   destination is on a different subnet than the source, the cut-through
   typically is to the egress router that serves the destination subnet.

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   The advantage of cut-through in such a case is that it avoids the
   latency of multiple router hops, and reduces load on "backbone"
   routers.  The cut-through decision is usually made by an entry router
   that is aware of both the routed and switched environments.

   This entry router communicates with a address resolution server using
   the Next Hop Resolution Protocol (NHRP) [Cansever] [Katz].  This
   server maps the destination network address to either a next-hop
   router (where cut-through is not appropriate) or to an egress router
   reached over the switched service.  Obviously, the data base in such
   a server may be affected by renumbering.  Clients may have a hard-
   coded address of the server, which again may need to change.

   While the NHRP work is in progress at the time of this writing,
   commercial implementations based on drafts of the protocol standard
   are in use.

4.  Numbering and Renumbering

   What is the role of any numbering plan?  To understand the general
   problem, it can be worthwhile to review the basic principles of
   routers.  While most readers will have a good intuitive sense of
   this, the principles have refined in the current usage of IP.

   A router receives an inbound IP datagram on one of its interfaces,
   and examines some number of bits of the destination address.  The
   sequence of bits examined by the router always begin at the left of
   the address (i.e., the most significant bit).  We call this sequence
   a "prefix."

   Routing decisions are made on totalPrefix bits, which start at the leftmost
   (i.e., most significant) bit position of the IP address.  Those totalPrefix
   bits may be completely under the control of the enterprise (e.g.,
   if they are in the private address space), or the enterprise may
   control the lowOrderPrefix bits while the highOrderPrefix bits are
   assigned by an outside organization.

   The router looks up the prefix in its routing table (formally called
   a Forwarding Information Base).  If the prefix is in the routing
   table, the router then selects an outgoing interface that will take
   the routed packet to the next hop IP address in the end-to-end route.
   If the prefix cannot be found in the routing table, the router
   returns an ICMP Destination Unreachable message to the source address
   in the received datagram.

   Assuming the prefix is found in the routing table, the router then
   transmits the datagram through the indicated outgoing interface. If
   multicast routing is in effect, the datagram may be copied and sent
   out multiple outgoing interfaces.

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4.1  Categorizing the topology

   From the router renumbering perspective, renumbering impact is apt to
   be greatest in highly connected parts of "backbones," and least in
   "stub" parts of the routing domain that have a single route to the

                         Global Internet
                            |                       |
                      +-----------+              +----------+
                      |           |              |          |
                    Reg1.1------Reg1.2          Reg2.1-----Reg2.2
                    |           |               |          |
                    |           |               |          |
                  Branch       Branch         Branch      Branch
                  1.1.1 to     1.2.1 to       2.1.1 to    2.2.1 to
                  1.1.N        1.2.N          2.1.N       2.2.N

   In this drawing, assume Back1 and Back2 exchange full routes; Back1
   is also the exterior router.  Regional routers (Reg) exchange full
   routes with one another and aggregate addresses to the backbone
   routers.  Branch routers default to regional routers.

   From a pure topological standpoint, the higher in the hierarchy, the
   greater are apt to be the effects of renumbering.  This is a first
   approximation to scoping the task, assuming addresses have been
   assigned systematically.  Systematic address space is rarely the case
   in legacy networks.

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4.2  Categorizing the address space

   An inventory of present and planned address space is a prerequisite
   to successful renumbering.  Begin by identifying the prefixes in or
   planned into your network, and whether they have been assigned in
   a systematic and hierarchical manner.

       +--Unaffected by renumbering [A]
       +--Existing prefixes to be renumbered
       |  |
       |  |
       |  +----To be directly renumbered on "flag day"
       |  |
       |  |
       |  +----Initially to be renumbered to temporary address
       +--Existing prefixes to be retired
       +--Planned new prefixes
          +---totalPrefix change, no length change
          +---highOrderPart change only, no length change
          +---lowOrderPart change only, no length change
          +---highOrderPart change only, high length change
          +---lowOrderPart change only, low length change
          +---totalPrefix change only, changes in high and low
          +---highOrderPart change only, no length change

   Ideally, a given prefix should either be "unchanged," "old," or
   "new." Renumbering will be easiest when each "old" prefix can be
   mapped to a single "new" prefix.

   Unfortunately, the ideal often will not be attainable.  It may be
   necessary to run parts of the new and old address spaces in parallel.

   Renumbering applies first to prefixes and then to host numbers to the
   right of the prefix.  To understand the scope of renumbering, it is
   essential to:

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      1.  Identify the prefixes (and possibly host fields) potentially
          affected by the renumbering operation.

      2.  Identify the authority that controls the values of the prefix,
          or part of the prefix, affected by renumbering.

   In a given enterprise, prefixes may be present that will be under
   the complete or partial control of the enterprise, as well as totally
   outside the control of the enterprise.  Let us review the principles
   of control over address space.

   More commonly, the most significant bits of the prefix are assigned    to the
   enterprise by an address registry (e.g., InterNIC, RIPE, or    APNIC) or by an
   Internet Service Provider (ISP).  This assignment    of a value in the most
   significant bit positions historically has been called a "network number,"
   when the assigned high-order part is 8, 16, or 24 bits long.  More recent
   usage does not limit the assigned part to a byte boundary.  The preferred
   term for the assigned part is a "CIDR block" of a certain number of bits [RFC1518].

   The enterprise then extends the prefix to the right, creating    "subnets."
   It is critical to realize that routers make routing    decisions based on the
   total prefix of interest, regardless of    who controls which bits.  In other
   words, the router really doesn't know or care about subnet boundaries.

   The way to think about subnetting is that it creates a longer prefix. Even
   before CIDR, we collapsed multiple subnets into a single network number
   advertisement sent to external routers.   In a more general way, we now think
   of extending the prefix to the right as subnetting and collapsing it to the
   left as supernetting, aggregating, or summarizing.  Depending on the usage of
   subnetting or aggregation, different prefix lengths are significant at different
   router interfaces.

4.3  Renumbering Scope

   Prefixes may be taken from the private address space [RFC1918]  that is not
   routable on the global Internet.  Since these addresses are not routable on
   the global Internet, changing parts of private address space prefixes is an
   enterprise-local decision.

   If a prefix is totally outside the control of the enterprise, it is  external,
   and will be minimally affected by routing.   Potential interactions of external
   prefixes with enterprise renumbering include:

      1)  Inadvertent alteration or deletion  of external addresses as
          part of router reconfiguration.
      2)  Loss of connectivity to application servers inside the
          enterprise, because the external client no longer knows the
          internal address of the server.
      3)  DNS/BGP
      4)  Security

   Prefixes partially under the control of the enterprise may change.
   The scope of this will vary depending on whether only the externally
   controlled part of the prefix changes, or if part of the internally
   controlled part is to be renumbered.  If the length of either the
   high-order or low-order parts change, the process becomes more complex.

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   High-order-part-only renumbering is most common when an organization
   changes ISPs, and needs to renumber into the new provider's space.
   The old prefix may have been assigned to the enterprise but will no
   longer be used for global routing, or the old prefix may have
   been assigned to the previous provider.  Note that administrative
   procedures may be necessary to return the previous prefix, although
   this usually will be done by the previous provider.  There often will
   need to be a period of coexistence between the old and new prefixes.

   Low-order-part-only renumbering can occur when an enterprise
   modifies its internal routing structure, and the changes only affect
   the internal subnet structure of the enterprise network. This
   is typical of efforts involved in increasing the number of available
   subnets (e.g., for more point-to-point media) or increasing the
   number of hosts on a medium (e.g., in greater use of workgroup

   Both the high-order and low-order parts may change.  This might
   happen when the enterprise changes to a new ISP, who assigns address
   space from a CIDR block rather than a classful network previously
   used.  With a different high-order prefix length, the enterprise
   might be forced to change its subnet structure.

5. Moving toward a Renumbering-Friendly Enterprise

   Renumbering affects both the configuration of specific router
   "boxes," and the overall system of routers in a routing domain.  The
   emphasis of this section is on making the current enterprise more
   renumbering-friendly, before any prefixes are actually changed.

   Renumbering will have the least impact when the minimum number of
   reconfiguration options are needed.  When planning renumbering on
   routers, consider that many existing configurations may contain
   hard-coded IP addresses that may not be necessary, even if
   renumbering were not to occur.  Part of a router renumbering effort
   should include, wherever possible, replacing router mechanisms
   based on hard-coded addresses with more flexible mechanisms.

   Renumbering will also generally be easier if the configuration
   changes can be made offline on appropriate servers, and then
   downloaded to the router if the router implementation permits.

5.1  Default Routes

   A well-known method for reducing the amount of reference by one
   router to other routers is to use a default route to a higher-level,
   better-connected router.  This assumes a hierarchical network
   design, which is generally desirable in the interest of scaling.

   Default routes are most appropriate for stub routers inside a routing domain,
   and for boundary routers that connect the domain to a single ISP.

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5.2  Route Summarization and CIDR

   When routes need to be advertised, summarize as much as is practical.
   Summarization is most effective when address prefixes have been
   assigned in a consistent and contiguous manner, which is often not
   the case in legacy networks.  Nevertheless, there is less to change
   when we can refer to blocks of prefixes.

   Not all routing mechanisms support general summarization.  Interior
   routing mechanisms that do include RIPv2, OSPF, EIGRP, IS-IS, and
   systems of static routes.  RIPv1 and IGRP do support classful
   summarization (i.e., at Class A/B/C network boundaries only).

   If existing addresses have been assigned hierarchically, it may be
   possible to renumber below the level of summarization, while hiding
   the summarization to the rest of the network.  In other words, if
   all the address bits being renumbered are to the right of the
   summarized prefix length, the change can be transparent to the
   overall routing system.

   Even when effective summarization is possible to hide the details of
   routing, DNS, filters, and other services may be affected by any

5.3  Server References in Routers

   Routers commonly communicate with an assortment of network management
   and other infrastructural servers.  Examples of these servers
   are given in the "Network Management" section below.  DNS itself,
   however, may be an important exception.

   Wherever possible, servers should be referenced by DNS name
   rather than by IP address.  If a specific router implementation only
   supports explicit address  references, this should be documented as
   part of the renumbering  plan.

   Routers may also need to  forward end host broadcasts to other
   infrastructure services (e.g., DNS, DHCP/BOOTP).  Configurations
   that do this are likely to contain hard-coded IP addresses of the
   destination hosts or their subnets, which will need to be changed
   as part of renumbering.

5.4  DNS and Router Renumbering

   The Domain Name Service is a powerful tool in any renumbering effort,
   and can help routers as well as end hosts.  If traceroute displays
   DNS names rather than IP addresses, certain debugging options can
   be transparent through the address transition.

   Be aware that dynamically learned names and addresses may be cached
   in router tables.  For a router to learn changes in address to name
   correspondence, it may be necessary to restart the router or
   explicitly clear the cache.

   Alternatively, router configuration files may contain hard-coded address/name
   correspondences that will not be affected by a change in the DNS server.

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   Different DNS databases are affected by renumbering.  For example,
   the enterprise usually controls its own "forward" data base, but
   the reverse mapping data base may be maintained by its ISP.  This
   can require coordination when changing providers.

   Commonly, router renumbering goes through a transition period.
   During this transition, old and new addresses may coexist in the
   routing system.  Coexistence over a significant period of time is
   especially likely for DNS references to addresses that are known
   in the global Internet [deGroot].  Various DNS servers throughout the
   world may cache addresses for periods of days.

   If, for example, a given router interface may have a coexisting
   new and old address, it can be appropriate to introduce the new
   address as a CNAME alias for the new address.

   DNS RR statements can end with a semicolon, indicating the rest of
   the line is a comment.  This can be used as the basis of tools to
   renumber DNS names for router addresses, by putting a comment
   (e.g., ";newaddr") at the end of the CNAME statements for the new
   addresses.  At an appropriate time, a script could generate a new
   zone file in which the new addresses become the primary definitions
   on A records, and the old addresses could become appropriately
   commented CNAME records.  At a later time, these commented CNAME
   entries could be removed.

   Care should be taken to assure that PTR reverse mapping entries
   are defined for new addresses, because some router vendor tools
   depend on reverse mapping.

5.5  Dynamic Addressing

   Renumbering is easiest when addresses need to be changed in the
   least possible number of places.  Dynamic address assignment is
   especially attractive for end hosts, and routers may play a key
   role in this process.  Routers may act as servers and actually
   assign addresses, or may be responsible for forwarding end host
   address assignment requests to address assignment servers.

   The most common use of dynamic address assignment is to provide
   IP addresses to end systems.  Dynamic address assignment, however,
   is also used to assign IP addresses to router interfaces.  An address
   assignment server may assign an IP address to a router either in the
   usual DHCP way, based on a MAC address in the router, or simply based
   on the physical connectivity of the new router.  In other words, any
   router connected on a specific interface of the configuring router
   would be assigned the same IP address.

   5.5.1 Router Roles in LAN-based DHCP Address Assignment

   End hosts attached to LANs often obtain address assignments from
   BOOTP or DHCP servers.  If the server is not on the same medium as
   the end hosts, routers may need to play a role in establishing
   connectivity between the end host and the address server.

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   If the client is not on the same medium as the address assignment
   server, routers either must act as address assignment services, or
   forward limited broadcasts to the location of appropriate servers.

   If the router acts as an address assignment server, its database
   of addresses that it can assign may change during renumbering.
   If the router forwards to a DHCP or BOOTP server, it must know the
   address of that server.

   While the usual perception of DHCP is that it assigns addresses
   from a pool, such that assignments to a given host at a given time
   is random within the pool, DHCP can also return a constant IP address
   for a specific MAC address.  This may be much easier to manage and
   troubleshoot, especially during renumbering.

   Clearly, if the DHCP server identifies end hosts based on their MAC
   address, consideration must be given to making that address unique,
   and changing the DHCP database if either the MAC address or the IP
   address changes.  One way to reduce such reconfiguration is to use
   Locally-Administered Addresses (LAA) on end hosts, rather than
   globally unique addresses stored in read-only memory (ROM).  Using
   LAAs solves the problem of MAC addresses changing when a network
   interface card changes, but LAAs have their own management problems
   of configuration into end systems and maintaining uniqueness within
   the enterprise.

5.5.2 Router Roles in Dialup Address Assignment

   There are several possible ways in which an dialup end host interacts
   with address assignment.  Routers/access servers can play critical
   roles in this process, either to provide connectivity between client
   and server, or directly to assign addresses.

   Different vendors handle address assignment in different ways.
   Methods include:

      1.  The access server forwards the request to a DHCP server, using
          a unique 48-bit identifier associated with the client.  Note
          that this usually should not be the MAC address of the access
          server, since the same MAC address would then be associated
          with different hosts.

      2.  The server forwards the request to an authentication server,
          which in turn gets a dynamic address either from:

             a.  internal pools
             b.  a DHCP server to which it forwards

     3.  The server selects an address from locally configured pools
         and provides it to the dialing host without the intervention
         of other services.

   When the router contains assignable addresses, these may need to
   change as part of renumbering.  Alternatively, hard-coded references
   to address assignment or authentication servers may need to change.

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   5.5.3 Router Autoonfiguration

   This initial address assignment may provide an address only for a
   single connection (i.e., between the new and configuring routers).
   The newly assigned address may then be used to "bootstrap" a full
   configuration into the new router.

   Dynamic address assignment to routers is probably most common
   at outlying "stub" or "edge" routers that connect via WAN links
   to a central location with a configuration server.  Such edge
   devices may be shipped to a remote site, plugged in to a WAN
   line, and use proprietary methods to acquire part or all of their
   address configuration.

   When such autoconfiguration is used on edge routers, it may be
   necessary to force a restart of the edge router after renumbering.
   Restarting may be the only way to force the autoconfigured router
   to learn its new address.  Other out-of-band methods may be available
   to change the edge router addresses.

5.6  Network Address Translation

   Network address translation (NAT) is a valuable technique for
   renumbering, or even for avoiding the need to renumber significant
   parts of an enterprise [RFC1631].  It is not always transparent to
   network layer protocols, upper layer protocols, and network
   management tools, and must not be regarded as a panacea.

   In the classic definition of NAT, certain parts of the routing
   system are designated as stub domains, and connect to the global
   domain only through NAT functions.  The NAT contains a translation
   mechanism that maps a stub address to a global address.  This
   mechanism may map statically or dynamically.

   A more general NAT mechanism often is implemented in firewall
   bastion hosts, which isolate "inside" and "outside" addresses
   through transport- or application-level authenticated gateways.
   Mappings from a "local" or "inside" address to a global
   address often is not one-to-one, because an inside address is
   dynamically mapped to a TCP or UDP port on an outside interface

   In general, if there are multiple NATs, their translation mechanisms
   should be synchronized.  There are specialized cases when a given
   stub address appears in more than one stub domain, and ambiguity
   develops if one wishes to map, say, from in stub
   domain A to in stub domain B.  In this case, both addresses identify different hosts.   Special mechanisms
   would have to exist to map the domain A local address into one
   global address, and to map the domain B local address into a
   different global address.

   NAT can have a valuable role in renumbering.  Its intelligent
   use can greatly minimize the amount of renumbering that needs
   to be done.  NAT, however, is not completely transparent.

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   Specifically, it can interfere with the proper operation of
   any protocol that carries an IP address as data, since NAT
   does not understand passenger fields and is unaware numbers
   need to change.

   Examples of protocols affected are:

      --TCP and UDP checksums that are in part based on the
        IP header.   This includes end-to-end encryption schemes
            that include the TCP/UDP checksum
      --ICMP messages containing IP addresses
      --DNS queries that return addresses or send addresses
      --FTP interactions that use an ASCII-encoded IP address
        as part of the PORT command

   It may be possible to avoid conflict if only certain hosts
   use affected protocols.  Such hosts could be assigned only
   global addresses, if the network topology and routing plan

   Early NAT experiments suggested that it needs a sparse end-to-end
   traffic mapping database for reasonable performance.  This may
   or may not be an issue in more hardware-based NAT implementations.

   Another concern with NAT is that unique host addresses are hidden
   outside the local stub domains.  This may in fact be desirable for
   security, but may present network management problems.  One
   possibility would be to develop a NAT MIB that could be queried
   by SNMP to find the specific local-to-global mappings in effect.

   There are also issues for DNS, DHCP, and other address management
   services.  There presumably would need to be local servers within
   stub domains, so address requests would be resolved uniquely in
   each stub (or would return appropriate global addresses).

6.  Potential Pitfalls in Router Renumbering

   One way to categorize potential pitfalls is to look at those associated
   with the overall numbering plan itself and routing advertisement, and
   those associated with protocol behavior.  In general, the former case
   is static and the latter is dynamic.

6.1  Static

   Problems can be implicit to the address/routing structure itself.
   These can include failures of components to understand arbitrary
   prefix addressing (i.e., classless routing), reachability due to
   inappropriate default or aggregated routes, etc.

6.1.1  Classless Routing Considerations

   Among the major reasons to renumber is to gain globally routable
   address  space.  In the global Internet, routable address space
   is based on arbitrary-length prefixes rather than traditional
   address classes.  Classless Inter-Domain Routing (CIDR) is the
   administrative realization of prefix addressing in the global

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   Internet.  Inside enterprises, arbitrary prefix length addressing
   often is called Variable Length Subnet Masking (VLSM) or
   "subnetting a subnet."

   The general rules of prefix addressing must be followed in CIDR.
   Among these are permitting use of the all-zeroes and all-one subnets
   [RFC1812], and not assuming that a route to a "Class A/B/C network number"
   implies routes to all
   subnets of that network.  Assumptions also should not be made that
   a prefix length implied by the structure of the high-order bits of
   the IP address (i.e., the "First Octet Rule").

   This ideal, unfortunately, is not understood by a significant number
   of routers (and terminal access servers that participate in routing),
   and an even more significant number of host IP implementations.

   When planning renumbering, network designers must know if the new address
   has been allocated using  CIDR rules rather than traditional classful
   addressing. If CIDR rules have been followed in address assignment, then steps
   need to be taken to assure the router understands them, or appropriate steps
   need to be taken to interface the existing environment to the CIDR

   Current experience suggests that it is best, when renumbering, to
   maintain future compatibility by moving to a CIDR-supportive routing
   environment.  While this is usually thought to mean introducing a
   classless dynamic routing protocol, this need not mean that routing
   become much more complex.  In a RIPv1 environment, moving to RIPv2
   may be a relatively simple change.  Alternative simple methods
   include establishing a default route from a non-CIDR-compliant
   routing domain to a CIDR-compliant service provider, or making use
   of static routes that are CIDR-compliant.

   Some routers support classless routing  without further
   configuration, other routers support classless routing but require
   specific configuration steps to enable it, while other routers only
   understand classful routing.  In general, most renumbering will
   eventually require classless routing support.  It is essential to
   know if a given router can support classless routing.  If it does
   not, workarounds may be possible.  Workarounds are likely to be
   necessary.  Aggregation Problems

   In experimenting with the CIDR use of a former Class A network
   number, it was shown in RFC1879 that CIDR compliance rules must
   be enabled explicitly in some routers, while other routers do not
   understand CIDR rules.

   RFC 1897 demonstrated problems with some existing equipment,
   especially "equipment that depends on use of a classful routing
   protocol, such as RIPv1 are prone to misconfiguration.  Tested
   examples are current   Ascend and Livingston gear, which continue
   to use RIPv1 as the default/only routing protocol.  RIPv1 use
   will create an aggregate announcement.... The Ascend was told to
   announce 39.1.28/24, but since RIPv1 can't do this, the Ascend
   instead sent 39/8."  RIPv1, like all classful interior protocols,
   is obsolescent.

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INTERNET-DRAFT               Router Renumbering Guide       August 1996  Discontiguous Networks

   Another problem that can occur with routers or routing mechanisms
   that do not understand arbitrary length prefix addressing is that
   of discontiguous networks.   This problem is easy to create
   inadvertently when renumbering.  In the example below, assume the
   enterprise has been using as its primary prefix, but
   has introduced an ISP whose registered addresses were in

   Assume a RIPv1 system of three routers:

                       |                  |
                       |                  |
             |               Router 1              |
             |               Router 2              |<------OUTSIDE
             |               Router 3              |
                       |                  |
                       |                  |

  Router 1 can reach its two locally connected subnets,
  and  It will aggregate them into a single announcement
  of when it advertises out the interface.

  In like manner, Router 3 can reach its two locally connected subnets, and  It will aggregate them into a single
  announcement of when it advertises out the

  When Router 2 receives a packet from its "outside" interface destined,
  say, to, where does it send it?  Router 2 has received
  two advertisements of on different interfaces, without any
  detail of subnets inside  Router 2 has an ambiguous routing
  table in terms of the next hop to a subnet of  We call this
  problem, when parts of the same classful network are separated by
  different networks, discontigous subnets.

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  Two problems occur in this configuration.  Router 2 does not know
  where to send outside packets destined for a subnet of
  Connectivity, however, also will break between Routers 1 and 3,
  because Router 2 does not know the next hop for any subnet of

  There are several workarounds to this problem.  Obviously, one would
  be to change to a routing mechanism that does advertise subnets.  An
  alternative would be to establish an IP-over-IP tunnel through Router
  2, and give this a subnet in  This additional subnet would
  be visible only in Routers 1 and 3.  It would solve the connectivity
  problem between Routers 1 and 3, but Router 2 would still not be able
  to forward outside packets.  This might be a perfectly acceptable
  solution if Router 2 is simply being used to connect two parts of

  Another way to deal with the discontiguous network problem is to
  assign secondary addresses in to the R1-R2 and R2-R3
  interfaces, which would allow the subnets to be advertised
  to R2.  This would work as long as there is no problem in advertising
  the subnets into the R2 routing system.  There would be a
  problem, for example, if the address were in the private
  address space but the R2 primary addresses were registered, and
  R2 advertised the addresses to the outside.

  This problem can be handled if R2 has filtering mechanisms that can
  selectively block advertisements to the outside world.
  The configuration, however, will become more and more complicated.  Router-Host Interactions

   The situation may not be as bleak if hosts do not understand prefix
   addressing but routers do.  Methods exist for hiding a VLSM
   structure from end hosts that do not understand it.  These do
   involve protocol mechanisms as workarounds, but the fundamental
   problem is hosts' understanding of arbitrary prefix lengths.

   A key mechanism is proxy ARP [Carpenter].  The basic mechanism of
   using proxy ARP in prefix-based renumbering is to have the hosts
   issue an ARP whenever they want to communicate with a destination.
   If the destination is actually on the same subnet, it will respond
   directly to the ARP.  If the destination is not, the router will
   respond as if it were the destination, and the originating host will
   send the datagram to the router.  Once the datagram is in the router,
   the VLSM-aware router can forward it.

   Many end hosts, however, will only issue an ARP if they conclude
   the destination is on their own subnet.  All other traffic is sent
   to a hard-coded default router address.  In such cases, workarounds
   may be needed to force the host to ARP for all destinations.

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   One workaround involves a deliberate misconfiguration of hosts.
   Hosts that only understand default routers also are apt only to
   understand classful addressing.  If the host is told its major
   (i.e., classful) network is not subnetted, even though the address
   plan actually is subnetted, this will often persuade it to ARP to
   all destinations.

   This also works for hosts that do not understand subnetting at all.
   An example will serve for both levels of host understanding.  Assume
   the enterprise uses as its major address, which is in
   the Class B format.  This is actually subnetted with eight bits of
   subnetting --  Individual hosts unaware of VLSM,
   however, either infer Class B from the address value, or are told
   that the subnet mask in effect is

   Yet another approach that helps hosts find routers is to run passive
   RIP on the hosts, so that they hear routing updates.  They assume any
   host that issues routing updates must be a router, so traffic for non-
   local destinations can be forwarded there.  While RIPv1 does not support
   arbitrary prefixes, the router(s) issuing the routing updates may have
   additional capabilities that let them correctly forward such traffic.
   The priority, therefore, is to get the non-local routers to a router that
   understands the overall routing structure, and passive RIP may be a
   viable way to do this.

   It may be appropriate to cut over on a site-by-site basis [Lear].  In
   such an approach, some sites have VLSM-aware hosts but others do not.
   As long as the routing structure supports VLSM, workarounds can be
   applied where needed.

6.1.2  MAC Address Interactions

   While it is uncommon now for a router to acquire any of its interface
   addresses as a DHCP client, this may become more common. When a
   router so acquires addresses, care must be taken that the MAC address
   presented to the DHCP server is in fact unique.

   Modern routers usually support protocol architectures besides IP.
   Some of these architectures, notably DECnet, Banyan VINES, Xerox
   Network Services, and IPX, may modify MAC addresses of routers such
   that a given MAC address appears on more than one interface.  While
   this is normally not a problem, it could cause difficulties when
   the MAC address is assumed to be unique.

   Other situations occur when the same MAC address can appear on more
   than one interface.  There are high-availability IBM options which
   establish primary and backup instances of the same MAC address on
   different physical interfaces of 37x5 communications controllers.

   Some end hosts running protocol stacks other than IP, notably
   DECnet, may change their MAC address from the globally assigned
   built-in address.

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6.2  Dynamic

   Dynamic protocol mechanisms that to some extent depend on IP
   addresses may be affected by router renumbering.  These include
   mechanisms that assign or resolve addresses (e.g., DHCP, DNS),
   mechanisms that use IP addresses for identification (e.g., SNMP),
   security and authentication mechanisms, etc.  The listed mechanisms
   are apt to have configuration files that will be affected by

   Another area of dynamic behavior that can be affected is caching.
   Application servers, directory functions such as DNS, etc., may
   cache IP addresses.  When the router is renumbered, these servers
   may point to old addresses.  Certain proxy server functions may
   reside on routers, and the router may need to be restarted to reset
   the cache.

   The endpoints of TCP tunnels terminating on routers may be internally
   identified by address/port pairs at each end.  If the address
   changes, even if the port remains the same, the tunnel is likely to
   need to be reestablished.

7.  Tools and Methods for Renumbering

   The function of a renumbering tool can be realized either as
   manual procedures as well as software. This section deals with
   functionality of hypothetical yet general renumbering tools rather
   than their implementation.

   General caveat:  tools should know whether the environment supports
   classless addressing.  If it does not, newly generated addresses
   should be checked to see they do not fall into the all-zeroes or
   all-ones subnet values.

7.1  Search Mechanisms

   Tools will be needed to search configuration files and other
   databases to identify addresses and names that will be affected by
   reorganization.  This search should be based on the address
   inventory described above.

   Especially when searching for names, common search tools using
   regular expressions (e.g., grep) may be very useful.  Some additional
   search tools may be needed. One would convert dotted decimal search
   arguments to binary and only then makes the comparison.

   The comparison may need to be done under the constraint of a mask.
   Such a comparator would also be relevant as the second phase that
   looks for ipAddress and other relevant strings in MIBs.

7.2  Address Modification

   The general mechanism for address modification is substitution of
   an arbitrary number of bits in an address.  In the simplest cases,
   there is a one-to-one correspondence between old and new bit

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   Especially when address modification is manual, it should be
   remembered that the affected bits can be obscured by dotted
   decimal notation.  Work in, or at least consider, binary notation
   to assure accuracy.

   Several basic functions can be defined:

      clear <length> bits starting at <position>
      OR the bit string <newbits> of <length> starting at <position>

   In these examples, the index [j] is used to identify entries in
   the address inventory database for existing addresses, while [k]
   identifies new addresses.

   Remember that these tools operate at a bit level, so the new
   address will have to be converted back into dotted decimal, MIB
   ASN.1, or other notation before it can be replaced into
   configuration files.

7.2.1  Prefix Change, No Change in Length

   If the entire new prefix has the same
   number of bits as the old external part, requiring no change to
   subnetting, the router part of renumbering may be fairly simple.
   If the router configurations are available in machine-readable form,
   as text files or parsable SNMP data, it is a relatively simple task
   to define a tool to examine IP addresses in the configuration,
   identify those beginning with the old prefix, and substitute the
   new prefix bit-by-bit.

   for each address[j],

   Note that the host part is unaffected.  Both subnet specifications
   (e.g., for route advertisements) and specific host references will
   be changed correctly in this simple case.

7.2.2  highOrderPart change

   Matters are slightly more complex when the change applies only to
   the externally-controlled part of the prefix, as might be the case
   when an organization changes ISPs and renumbers into the ISP's
   address space, without changing the internal subnet structure.

   The substitution process is much as the previous case, except only
   the high-order bits change:

   for each address,

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7.2.3  lowOrderPart change

   Organizations might renumber only the lowOrderPart (i.e., the subnet
   field) of their
   address space.  This might be done to clean up an address space
   into contiguous blocks prior to introducing a routing system
   that aggregates addresses, such as OSPF.

   for each address[j],

7.2.4  lowOrderPart change, Change in lowOrderPart length

   When the length of the subnet field changes in all or part of the
   address space, things become significantly more complex. A fairly
   simple case arises when the host field is consistently too long,
   at least in the affected subnets.  This is common, for example,
   when address space is being recovered in an existing Class B
   network with 8 bits of subnetting.  Certain /24 bit prefixes are
   being extended to /30 and reallocated to point-to-point real or
   virtual (e.g., DLCI) media.

   for each address [j]
    if address is affected by renumbering
     if newLowOrderPartLength[k] > oldLowOrderPartLength[j]

7.2.5  highOrderPart change, Change in highOrderPart length

  When the length of the high-order part changes, but it is desired to
  keep the existing subnet structure, constraints apply. The situation
  is fairly simple if the new high-order part is shorter than the
  existing high order part.

  If the new high-order part is longer than the old high order part,
  constraints are more complex.  The key is to see if any of the <k>
  most significant bits of the oldHighOrderPart, which overlap the <k>
  least significant bits of the newHighOrderPart, are nonzero.  If no
  bits are nonzero, it may be simply to overlay the new prefix bits.

7.3  Naming

   It is worthwhile to distinguish that a router's use of a DNS name
   does not necessarily mean that name is defined in a name server.
   Routers often contain static address to name mappings local to the
   router, so both the DNS zone files and the router configurations will
   need to be checked.

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   What we first want to do is generate a list of name/address mappings,
   the mapping mechanism for each instance (e.g., static entry in
   configuration file, RR in our zone's DNS, RR in a zone file outside
   ours), the definition statement (or equivalent if the routers are
   configured with SNMP), and current IP address.  We then want to
   compare the addresses in this list to the list defined earlier of
   prefixes affected by renumbering.   The intersection of these lists
   define where we need to make changes.

   Note that the explicit definition statement, or at leasts its
   variables, should be kept.  In the real world, static IP address
   mappings in hosts may not have been maintained as systematically
   as are RR records in a DNS server.   It is entirely possible that
   different host mapping entries for the same name point to different

7.3.1  DNS Tools

   The DNS itself can both delay and and speed router renumbering.
   Caches in DNS servers both inside and outside the organization may
   have sufficient persistence that a "flag day" cutover is not
   practical if worldwide connectivity is to be kept.  DNS can help,
   however, make a period of old and new address coexistence work.

   If, for example, a given router interface may have a coexisting
   new and old address, it can be appropriate to introduce the new
   address as a CNAME alias for the new address.

   DNS RR statements can end with a semicolon, indicating the rest of
   the line is a comment.  This can be used as the basis of tools to
   renumber DNS names for router addresses, by putting a comment
   (e.g., ";newaddr") at the end of the CNAME statements for the new
   addresses.  At an appropriate time, a script could generate a new
   zone file in which the new addresses become the primary definitions
   on A records, and the old addresses could become appropriately
   commented CNAME records.  At a later time, these commented CNAME
   entries could be removed.

   Care should be taken to assure that PTR reverse mapping entries
   are defined for new addresses, because some router vendor tools
   depend on reverse mapping.

7.3.2   Related name tools

   Especially on UNIX and othe that do routing, there may be static
   name definitions.  Such definitions are probably harder to keep
   maintained than entries in the DNS, simply because they are more
   widely distributed.

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   Several tools are available to generate more maintainable entries.
   A perl script called h2n converts host tables into zone data files
   that can be added to the DNS server.  It is available as
   Another tool, makezones, is part of the current BIND distribution,
   and can also be obtained from

   See the DNS Resources Directory at http://www.dns.net/dnsrd.

8.  Router Identifiers

   Configuration commands in this category assign IP addresses to
   physical or virtual interfaces on a single router. They also include
   commands that assign IP-address-related information to the router
   "box" itself, and commands which involve the router's interaction
   with neighbors below the full routing level (e.g., default gateways,

   Routers may have other unique identifiers, such as DNS names used
   for the set of addresses on the "box," or SNMP systemID strings.

8.1. Global Router Identification

   Traditional IP routers do not have unique identifiers, but rather are
   treated as collections of IP addresses associated with their
   interfaces.  Some protocol mechanisms, notably OSPF and BGP, need an
   address for the router itself, typically to establish tunnel
   endpoints between peer routers.  Other applications include
   "unnumbered interfaces" used to conserve address space for serial
   media, a practice discussed further below.

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   In the illustration below, the address space is used for
   global identifiers.  A TCP tunnel runs from to, but its
   traffic is load-shared among the two real links, and

                       |                  |
                       |                  |
             |               Router 1              |
             |                                     |
             |                |
             |                   #                 |
                |  #          |
                |                #          |
                |                #          |
                |                #          |
                |                #          |
                |                #          |
                |                #          |
                |  #          |
             |               Router 2              |
             |                                     |
             |                |
             |                                     |
                       |                  |
                       |                  |

   A common practice to provide router identifiers is using
   the "highest IP address" on the router as an identifier for the
   "box."  Many implementations have a default mechanism
   to establish the router ID, which may be the highest configured
   address, or the highest active address.

   Typical applications of a global router ID may not require it be
   a "real" IP address that is advertised through the routing domain,
   but is simply a 32-bit identifier local to each router.  When this
   is the case, this identifier can come from the RFC 1918 private
   address space rather than the enterprise's registered address space.

   Allowing default selection  of the router ID can be unstable and is
   not recommended.  Most implementations have a means of declaring
   a pseudo-IP address for the router itself as opposed to any of its

   Changes to this pseudo-address may have implications for DNS.  Even
   if this is not a real address, A and PTR resource records may have
   been set up for it, so diagnostics can display names rather than

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   Another potential DNS implication is that a CNAME may have been
   established for the entire set of interface addresses on a router.
   This allows testing, telnet, etc., to the router via any reachable

8.2  Interface Address

   Interface addresses are perhaps the most basic place to begin router
   renumbering.  Interface configuration will require an IP address, and
   usually a subnet mask or prefix length.  Some implementations may not
   have a subnet mask in the existing configuration, because they use a
   "default mask" based on a classful assumption about the address.
   Be aware of possible needs for explicit specification of a subnet
   mask or other prefix length specification when none previously
   was specified.  This will be especially common on older host-based

   Multiple IP addresses, in different subnets, can be assigned to the
   same interface.  This is often a valuable technique in renumbering,
   because the router interface can be configured to respond to both the
   new and old addresses.

   Caution is necessary, however, in using multiple subnet addresses on
   the same interface.  OSPF and IS-IS implementations may not advertise
   the additional addresses, or may constrain their advertisement so all
   must be in the same area.

   When this method is used to make the interface respond to new and
   old addresses, and the renumbering process is complete, care must
   be taken in removing the old addresses.  Some router implementations
   have special meaning to the order of address declarations on an
   interface.  It is highly likely that routers, or at least the
   interface, must be restarted after an address is removed.

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8.3  Unnumbered Interfaces

   As mentioned previously, several conventions have been used to avoid
   wasting subnet space on serial links.  One mechanism is to implement
   proprietary "half-router" schemes, in which the unnumbered link
   between router pairs is treated as an "internal bus" creating a
   "virtual router," such that the scope of the unnumbered interface
   is limited to the pair of routers.

   |             +------------+                +------------+       |
   |             |            |                |            |       |
   |          e0 |            |s0           s0 |            |       |
   |-------------|     R1     |................|     R2     |-------|
   | ||                ||       |
   |      /24    |            |                |            |       |
   |             +------------+                +------------+

   In the above example, software in routers R1 and R2 automatically
   forward every packet received on serial interface S0 to Ethernet
   interface E0.  They forward every packet on e0 to their local S0.
   Neither S0 has an IP address.  R1 has the router ID
   and R2 has

   It is thus impossible to send a specific ping to the S0 interfaces,
   making it difficult to test whether a connectivity problem is due
   to S0 or E0.  Some management is possible as long as at least one
   IP address on the router (e.g., E0) is reachable, since this will
   permit SNMP connectivity to the router.  Once the router is reachable
   with SNMP, the unnumbered interface can be queried through the
   MIB ifTable.

   Another approach is to use the global router identifier as a pseudo-
   address for every unnumbered interface on a router.  In the above
   example, R1 would use as its identifier.  This provides an
   address to be used for such functions as the IP Route Recording
   option, and for providing a next-hop-address for routes.

   The second approach is cleaner, but still can create operational
   difficulties.  If there are multiple unnumbered interfaces on
   a router, which one (if any) should/will respond to a ping?  Other
   network management mechanisms do not work cleanly with unnumbered

   As part of a renumbering effort, the need for unnumbered interfaces
   should be examined.  If the renumbering process moves the domain
   to classless addressing, then serial links can be given addresses
   with a /30 prefix, which will waste a minimum of address space.

   For dedicated or virtual dedicated point-to-point links within an
   organization, another alternative to unnumbered operation is using
   RFC1918 private address space.  Inter-router links rarely need to
   be accessed from the Internet unless explicitly used for exterior

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   If unnumbered interfaces are kept, and the router-ID convention
   is used, it will probably be more stable to rely on an explicitly
   configured router ID rather than a default from a numbered interface

   The situation becomes even more awkward if it is desired to use
   unnumbered interfaces over NBMA services such as Frame Relay.  OSPF,
   for example, uses the IP address of numbered interfaces as a unique
   identifier for that interface.  Since unnumbered interfaces do not
   have their own unique address, OSPF has not obvious way to identify
   these interfaces.  A physical index (e.g., ifTable) could be used,
   but would have to be extended to have an entry for each logical
   entry (i.e., VC) multiplexed onto the physical interface.

8.4  Address Resolution

   While mapping of IP addresses to LAN MAC addresses is usually done
   automatically by the router software, there will be cases where
   special mappings may be needed.  For example, the MAC address used by
   router interfaces may be locally administered (i.e., set manually),
   rather than relying on the burnt-in hardware address.  It may be part
   of a proprietary  method that dynamically assigns MAC addresses to
   interfaces.  In such cases, an IP address may be part of the MAC
   address configuration statements and will need to be changed.

   Manual mapping to medium addresses will usually be needed for
   NBMA and switched media.  When renumbering IP addresses, statements
   that map the IP address to frame relay DLCIs, X.121 addresses,
   SMDS and ATM addresses, telephone numbers, etc., will need to
   be changed to the new address.  Local requirements may
   require a period of parallel operation, where the old and new
   IP addresses map to the same medium address.

8.5  Broadcast Handling

   RFC1812 specifies that router interfaces MUST NOT forward limited
   broadcasts (i.e., to the all-ones destination address,  It is common, however, to have circumstances
   where a LAN segment is populated only by clients that communicate
   with key servers (e.g., DNS or DHCP) by sending limited broadcasts.
   Router interfaces can cope with this situation by translating
   the limited broadcast address to a directed broadcast address or
   a specific host address, which is legitimate to forward.

   When limited address translation is done for serverless segments,
   and the new target address is renumbered, the translation rule
   must be reconfigured on every interface to a serverless segment.
   Be sure to recognize that a given segment might have a server
   from the perspective of one service (e.g., DHCP), but could be
   serverless for other services (e.g., NFS or DNS).

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8.6  Dynamic Addressing Support

   Routers can participate in dynamic addressing with RARP, DHCP,
   BOOTP, or PPP.   In a renumbering effort, several kinds of
   changes may made to be made on routers participating in dynamic

   If the router acts as a server for dynamic address assignment,
   the addresses it assigns will need to be renumbered.   These
   might be specific addresses associated with MAC addresses or
   dialup ports, or could be a pool of addresses.  Pools of addresses
   may be seen in pure IP environments, or in multiprotocol
   situations such as Apple MacIP.

   If the router does not assign addresses, it may be responsible for
   forwarding address assignment requests to the appropriate server(s).
   If this is the case, there may be hard-coded references to the IP
   addresses of these servers, which may need to be changed as part
   of renumbering.

9. Filtering and Access Control

   Routers may implement mechanisms to filter packets based on criteria
   other than next hop destination.  Such mechanisms often are
   implemented differently for unicast packets (the most common case)
   or for multicast packets (including routing updates).  Filtering
   rules may contain source and/or destination IP addresses
   that will need to change as part of a renumbering effort.

   Filtering can be done to implement security policies or to
   control traffic.  In either case, extreme care must be taken
   in changing the rules, to avoid leakage of sensitive information.
   denial of access to legitimate users, or network congestion.

   Routers may implement logging of filtering events, typically
   denial of access.  If logging is implemented, logging servers
   to which log events are sent preferably should be identified
   by DNS name.  If the logging server is referenced by IP address,
   its address may need to change during renumbering.   Care should
   be taken that critical auditing data is not lost during the
   address change.

9.1  Static Access Control Mechanisms

   Router filters typically contain some number of include/exclude
   rules that define which packets to include in forwarding and
   which to exclude.  These rules typically contain an address
   argument and some indication of the prefix length.  This length
   indication could be a count, a subnet mask, or some other mask.

   When renumbering, the address argument clearly has to change.
   It can be more subtle if the prefix length changes, because
   the length specification in the rule must change as well. Needs
   for such changes may be hard to recognize, because they apply
   to ranges of addresses that might be at a level of aggregation
   above the explicit renumbering operation.

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   RFC 1812 requires that address-based filtering allow arbitrary
   prefix lengths, but some hosts and routers might only allow
   classful prefixes.

9.2  Special Firewall Considerations

   Routers are critical components of firewall systems.  Architecturally,
   two router functions are described in firewall models, the external screening
   router between the outside and the "demilitarized zone (DMZ)," and the internal
   screening router between the inside and the "perimeter network."  Between these
   two networks is the bastion host, in which reside various non-routing isolation
   and authentication functions, beyond the scope of this document.

   One relevant aspect of the bastion host, however, is that it may do address
   translation or higher-layer mappings between differnt address spaces.  If the
   "outside" address space (i.e., visible to the Internet) changes, this will mean
   that the outside screening router will need configuration changes.  Since the
   outside screening router may be under the control of the ISP rather than the
   entrerprise, administrative coordination will be needed.

                               DMZ  +--------+      Peri-
                                |---| Public |      meter
                +-----------+   |   |  Hosts |        |   +-----------+
   From         | External  |   |   +--------+        |---| Internal  |
   Internet.....| Screening |---|   +--------+        |   | Screening |
                | Router    |   |---| Bastion|--------|   | Router    |......To
                +-----------+   |   |  Host  |        |   +-----------+   Internal
                                |   +--------+        |   +-----------+    Network
                                |   +--------+        |---| Dialup    |
                                |---|  Split |        |   | Access    |
                                |   |  DNS   |        |   | Server    |
                                |   +--------+        |   +-----------+

   External screening routers typically have inbound access lists that block
   unauthorized traffic from the Internet, and outbound access lists that permit
   access only to DMZ servers and the bastion host.   The inbound filters commonly
   block the Private Address Space, as well as address space from the enterprise's
   internal network.  If the internal network address changes, the inbound filters
   clearly will need to change.

   If DMZ host addresses change, the corresponding outbound filters from the
   external screening host also will need to change.

   Internal screening routers permit access from the internal network to selected
   servers on the perimeter network, as well as to the bastion host itself.  If the
   enterprise uses private address space internally, renumbering may not affect this

   Another component of a firewall system is the "split DNS" server, which provides
   address mapping in relation to the globally visible parts of the

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9.3  Dynamic Access Control Mechanisms

   Certain access control services, such as RADIUS and TACACS+,
   may insert dynamically assigned access rules into router
   configurations.  For example, a RADIUS database "contains a list
   of requirements which must be met to allow
   access for the user.  This always includes verification of the
   password, but can also specify the client(s) or port(s) to which the
   user is allowed access. [Rigney]."

   Configuration information dynamically communicated to the router may
   be in the form of filtering rules.  Effectively, this authentication
   database becomes an extension of the router configuration database.
   Both these databases may need to change as part of a renumbering

   Another dynamic configuration issue arises when "stateful packet
   screening" on bastion hosts or routers is used to provide security
   for UDP-based services, or simply for IP.  In such services, when
   an authorized packet leaves the local environment to go into an
   untrusted address space, a temporary filtering rule is established
   on the interface on which the response to this packet is expected.
   The rule typically has a lifetime of a single packet response.
   If these rules are defined in a database outside of the router,
   the rule database again is an extension of router configuration that
   must be part of the renumbering effort.

10.  Interior Routing

   This section deals with routing inside an enterprise, which
   generally follows, ignoring default routes, the rules:

      1.  Does a single potential route exist to a destination?
          If so, use it.
      2.  Is there more than one potential path to a destination?
          If so, use the path with the lowest end-to-end metric.
      3.  Are there multiple paths with equal lowest cost to the
          destination?  If so, consider load balancing.

   Most enterprises do not directly participate in global Internet
   routing mechanisms, the details of which are of concern to their
   service providers.  The next section deals with those more complex
   exterior mechanisms.

10.1  Static Routes

   During renumbering, the destination and/or next hop address of
   static routes may need to change.  It may be necessary to restart
   routers or explicitly clear a routing table entry to force the
   changed static route to take effect.

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10.2  RIP (Version 1 unless otherwise specified)

   The Routing Information Protocol (RIP) has long been with us, as one of the
   first interior routing protocols.  It still does that job in small networks,
   and also has been used for assorted functions that are not strictly part of
   interior routing.  In this discussion, we will first deal with pure interior
   routing applications.

   In a renumbering effort that involves classless addressing, RIPv1
   may not be able to cope with the new addressing scheme.  Officially,
   this protocol is Historic and should be avoided in new routing
   plans.  Where legacy support requirements dictate it be retained,
   it is worthwhile to try to limit RIPv1 in "stub" parts of the network.
   Vendor-specific mechanisms may be available to interface RIPv1
   to a classless environment.

   As part of planning renumbering, strong consideration should be given
   to moving to RIPv2, OSPF, or other classless routing protocols as the primary
   means of interior routing.  Doing so, however, may not remove the need to run
   RIP in certain parts of the enterprise.

   RIP is widely implemented on hosts, where it may be used as a method of
   router discovery, or for load-balancing and fault tolerance when multiple
   routers are on a subnet.  In these applications, RIP need not be the only
   routing protocol in the domain; RIP may be present only on stub subnets.
   Destination information from more capable routing protocols may be translated
   into RIP updates.  While it is generally reasonable to minimize or remove RIP
   as part of a renumbering effort, be careful not to disable the ability of hosts
   to locate routers.

   RIP is also used as a quasi-exterior routing mechanism between some customers
   and their ISPs, as a means simpler than BGP for the customer to announce
   routes to the provider.

10.3  OSPF

   OSPF has several sensitivities to renumbering beyond those of
   simpler routing protocols.  If router IDs are assigned to be part
   of the registered address space, they may need to be changed as
   part of the renumbering effort.  It may be appropriate to use
   RFC 1918 private address space for router IDs, as long as these
   can be looked up in a DNS server within the domain.

   Summarization rules are likely to be affected by renumbering,
   especially if area boundaries change.

   Special addressing techniques, such as unnumbered interfaces and
   physical interfaces with IP addresses in multiple subnets, may
   not be transparent to OSPF.  Care should be exercised in their
   use, and their use definitely should be limited to intra-area

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   If part of the renumbering motivation is the introduction of
   NBMA services, there can be numerous impacts on OSPF.  Generally,
   the best way to minimize impact is to use separate subnets for
   each VC.  By doing so, different OSPF costs can be assigned to
   different VCs, designated router configuration is not needed, etc.

10.4  IS-IS

   IP prefixes are usually associated with IS-IS area definitions.
   If IP prefixes change, there may be a corresponding change in area

10.5  IGRP and Enhanced IGRP

   When a change from IGRP to enhanced IGRP is part of a renumbering
   effort, the need to disable IGRP automatic route summarization needs
   to be considered.  This is likely if classless addressing is being

   Also be aware of the nuances of automatic redistribution between
   IGRP and EIGRP.  The "autonomous system number," which need not be
   a true AS number but simply identifies a set of cooperating routers,
   must be the same on the IGRP and EIGRP processes for automatic
   redistribution to occur.

11.  Exterior Routing

   Exterior routes may be defined statically.  If dynamic routing is involved, such
   routes are learned primarily from BGP.  RIP is not infrequently used to allow
   ISPs to learn dynamically of new customer routes, although there are security
   concerns in such an approach.  IGRP and EIGRP can be used to advertise external

   Renumbering that affects BGP-speaking routers can be complex, because
   it can require changes not only in the BGP routers of the local Autonomous System,
   but also require changes in routers of other AS and in routing registries.  This
   will require careful administrative  coordination.

   If for no other reason than documentation, consider use of a routing policy
   notation [RIPE-181++] [RPSL] to describe exterior routing  policies

11.1  Routing Registries/Routing Databases

   Organizations who participate in exterior routing usually will
   have routing information not only in their routers, but in databases
   operated by registries or higher-level service providers (e.g.,
   the Routing Arbiter).

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   If an ISP whose previous address space came from a different
   provider either renumbers into a different provider's address space,
   or gains a recognized block of its own, there may be administrative
   requirements to return the previously allocated addresses.  These
   include changes in IN-ADDR.ARPA delegation, SWIP databases, etc.,
   and need to be coordinated with the specific registries and providers
   involved.   Not all registries and providers have the same policies.

   If the enterprise is a registered Autonomous System and  renumbers into a
   different address space, route objects with old prefixes in routing registries
   need to be deleted and route objects with new prefixes need to be added.

11.2  BGP--Own Organization

   IP addressing information can be hard-coded in several aspects
   of a BGP speaker.  These include:

      1.  Router ID
      2.  Peer router IP addresses
      3.  Advertised prefix lists
      4.  Route filtering rules

Some tools exist [RtConfig] for generating policy configuration part of BGP router
configuration statements from the policies specified in RIPE-181 or RPSL.

11.3  BGP--Other AS

   Other autonomous systems, including nonadjacent ones, can contain
   direct or indirect (e.g., aggregated) references to the above
   routing information.  Tools exist that can do preliminary checking
   of connectivity to given external destinations [RADB].

12.  Network Management

   This section is intended to deal with those parts of network
   management that are intimately associated with routers, rather than
   a general discussion of renumbering and network management.

   Methods used for managing routers include telnets to virtual console
   ports, SNMP, and TFTP.  Network management scripts may contain
   hard-coded references to IP addresses supporting these services.
   In general, try to convert script references to IP addresses to
   DNS names.

   A critical and complex problem will be converting SNMP databases,
   which are usually organized by IP address.

12.1  Configuration Management

   Names and addresses of servers that participate in configuration
   management may need to change, as well as the contents of the
   configurations they provide. TFTP servers are commonly used here,
   as may be SNMP managers.

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12.2  Name Resolution/Directory Services

   During renumbering, it will probably be useful to assign DNS names
   to interfaces, virtual interfaces, and router IDs of routers.
   Remember that it is perfectly acceptable to identify internal
   interfaces with RFC1597/RFC1918 private addresses, as long as
   firewalling or other filtering prevent these addresses to be
   propagated outside the enterprise.

   If dynamic addressing is used, dynamic DNS should be considered.
   Since this is under development, it may  be appropriate to consider
   proprietary means to learn what addresses have been assigned
   dynamically, so they can be pinged or otherwise managed.

   Also remember that some name resolution may be done by static tables
   that are part of router configurations.  Changing the DNS entries,
   and even restarting the routers, will not change these.

12.3  Fault Management

   Abnormal condition indications can be sent to several places that
   may have hard-coded IP addresses, such as SNMP trap servers,
   syslogd servers, etc.

   It should be remembered that large bursts of transient errors
   may be caused as part of address cutover in renumbering.  Be
   aware that these bursts might overrun the capacity of logging
   files, and conceivably cause loss of auditing information.
   Consider enlarging files or otherwise protecting them during

12.4  Performance Management

   Performance information can be recorded in routers themselves,
   and retrieved by network management scripts.  Other performance
   information may be sent to syslogd, or be kept in SNMP data bases.

   Load-generating scripts used for performance testing may contain
   hard-coded IP addresses.  Look carefully for scripts that contain
   executable code for generating ranges of test addresses.  Such
   scripts may, at first examination, not appear to contain explicit
   IP addresses.  They may, for example, contain a "seed" address used
   with an incrementing loop.

12.5  Accounting Management

   Accounting records may be sent periodically to syslogd or as SNMP
   traps.  Alternatively, the SNMP manager or other management
   applications may periodically poll accounting information in
   routers, and thus contain hard-coded IP addresses.

12.6  Security Management

   Security management includes logging, authentication, filtering,
   and access control.  Routers can have hard-coded references to
   servers for any of these functions.

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   In addition, routers commonly will contain filters containing
   security-related rules.  These rules are apt to need explicit
   recoding, since they tend to operate on a bit level.

   Some authentication servers and filtering mechanisms may dynamically
   update router filters.

12.7  Time Service

   Hard-coded references to NTP servers should be changed to DNS
   when possible, and renumbered otherwise.

13.  IP and Protocol Encapsulation

   IP packets can be routed to provide connectivity for non-IP
   protocols, or for IP traffic with addresses not consistent with
   the active routing environment.  Such encapsulating functions
   usually have a tunneling model, where an end-to-end connection
   between two "passenger" protocol addresses is mapped to a pair
   of endpoint IP addresses.   Generic Route Encapsulation is a
   representative means of such tunneling [RFC1701, RFC1702].

13.1  Present

   Renumbering of the primary IP environment often does not mean
   that passenger protocol addresses need to change.  In fact,
   such protocol encapsulation for IP traffic may be a very viable
   method for handling legacy systems that cannot easily be renumbered.
   For this legacy case, the legacy IP addresses can be tunneled over
   the renumbered routing environment.

   Also note that IP may be a passenger protocol over non-IP systems
   using IPX, AppleTalk, etc.

13.2  Future

   Tunneling mechanisms are fundamental for the planned transition of
   IPv4 to IPv6.  As part of an IPv4 renumbering effort, it may be
   worthwhile to reserve some address space for future IPv6 tunnels.

   While there are clear and immediate needs for IPv4 renumbering,
   there may be cases where IPv4 renumbering can be deferred for
   some months or years.  If the effort is deferred, it may be prudent
   at that time to consider if available IPv6 implementations or
   tunneling mechanisms form viable alternatives to IPv4 renumbering.
   It might be appropriate to renumber certain parts of the existing
   IPv4 space directly into the IPv6 space.  Tools for this purpose
   are experimental at the time this document was written.

14.  Security Considerations

   Routers are critical parts of firewalls, and are otherwise used for
   security enforcement.  Configuration errors made during renumbering
   can expose systems to malicious intruders, or deny service to
   authorized users.  The most critical area of concern is that filters
   are configured properly for old and new address, but other numbers
   also can impact security, such as pointers to authentication,
   logging, and DNS servers.

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   During a renumbering operation, it may be appropriate to introduce
   authentication mechanisms for routing updates.

15.  Planning and Implementing the Renumbering

   Much of the effort in renumbering will be on platforms other than
   routers.  Nevertheless, routers are a key part of any renumbering

   Step 1--Inventory of affected addresses and names.

   Step 2--Design any needed topological changes.  If temporary address
        space, network address translators, etc., are needed, obtain

   Step 3--Install and test changes to make the network more
        renumbering-friendly.  These include making maximum use of
        default routes  and summarization, while minimizing address-
        based references to servers.

   Step 4--Plan the actual renumbering.  Should it be phased or total?
        Can it be done in a series of stub network renumberings,
        possibly with secondary addresses on core routers?  Is NAT
        appropriate?  If so, how is it to be used?

        What is your plan of retreat if major problems develop?
        Make a distinction between problems in the routing system
        and unforeseen problems in hosts affected by renumbering.

   Step 5--Take final backups.

   Step 6--Cut over addresses and names, or begin coexistence.

        Make needed DNS and firewall changes.
        Restart routers and servers as appropriate.
        Clear caches as appropriate.
        Remember static name definitions in routers may not be affected
          by DNS changes.
        Coordinate changes with affected external organizations (e.g.,
          ISPs, business partners, routing registries)

   Step 6--Document what isn't already documented.  Make notes to help
        the person who next needs to renumber.  Share experience with
        the PIER working group or other appropriate organizations.

15.1  Applying Changes

   Renumbering changes should be introduced with care into operational
   networks.   For changes to take effect, it is likely that at least
   interfaces and probably routers will have to be restarted.  The
   sequence in which changes are applied must be carefully thought out,
   to avoid loss of connectivity, routing loops, etc., while the
   renumbering is in process.

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   See case studies presented to the PIER Working Group for examples
   of operational renumbering experience.  Organizations that have
   undergone renumbering have had to pay careful attention to informing
   users of possible outages, coordinating changes among multiple sites,
   etc.  It will be an  organization-specific decision whether router
   renumbering can be implemented incrementally or must be done in
   a major "flag day" conversion.

   Before making significant changes, TAKE BACKUPS FIRST of all router
   configuration files, DNS zone files, and other information that
   documents your present environment.

15.2  Configuration Control

   Operationally, an important part of renumbering and continued
   numbering maintenance is not to rely on local router interfaces,
   either command language interpreter, menu-based, or graphic, for
   the more sophisticated aspects of configuration, but to do primary
   configuration (and changes) on an appropriate workstation.  On
   a workstation or other general-purpose computer, configuration
   files can be edited, listed, processed with macro processors and
   other tools, etc.   Source code control tools can be used on the
   router configuration files.

   Once the configuration file is defined for a router, mechanisms
   for loading it vary with the specific router implementation.  In
   general, these will include a file transfer using FTP or TFTP
   into a configuration file on the router, SNMP SET commands,
   or logging in to the  router as a remote console and using a terminal
   emulator to   upload the new configuration under the router's interactive
   configuration mode.  Original acquisition of legacy configuration
   files is the inverse of this process.

15.3  Avoiding Instability

   Routing processes tend towards instability when they suddenly
   need to handle very large numbers of updates, as might occur if
   a "flag day" cutover is not carefully planned.  A general
   guideline is to make changes in only one part of a routing
   hierarchy at a time.

   Routing system design should be hierarchical in all but the
   smallest domains.  While OSPF and IS-IS have explicit area-based
   hierarchical models, hierarchical principles can be used with
   most implementations of modern routing protocols.  Hierarchy
   can be imposed on a protocol such as RIPv2 or EIGRP by judicious
   use of route aggregation, routing advertisement filtering, etc.

   Respecting a hierarchical model during renumbering means
   such things as renumbering a "stub" part of the routing
   domain and letting that part stabilize before changing other
   parts.  Alternatively, it may be reasonable to add new numbers
   to the backbone, allowing it to converge, renumbering stubs,
   and then removing old numbers from the backbone.  Obviously,
   these guidelines are most practical when there is a distinct
   old and new address space without overlaps.  If a block of
   addresses must simply be reassigned, some loss of service must
   be expected.

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16.  Acknowledgments

   Thanks to Jim Bound, Geert Jan de Groot, Paul Ferguson, Matt
   Holdrege, Dorian Kim,  Walt Lazear, Eliot Lear, Will Leland, and
   Bill Manning for advice and comments.

17.  References

   [ovrvw]  Work in progress; "Network Renumbering Overview:   Why would I want
      it and what is it anyway?"; , P. Ferguson, H. Berkowitz; PIER Working Group;
   [Cansever]  Work in progress, D. Cansever, "NHRP Protocol
      Applicability Statement"
   [Katz] Work in progress.  D. Katz, D. Piscitello, B. Cole, J.
      Luciani.  "Next Hop Resolution Protocol (NHRP)."
      K. Hubbard, J. Postel, M. Kosters, D. Conrad, D. Karrenberg;
   [RFC1631] "The IP Network Address Translator (NAT)"; K. Egevang,
      P. Francis; May 1994
   [RFC1918]Y. Rekhter,  R. Moskowitz, D. Karrenberg, G. de Groot, E. Lear,
      "Address Allocation for Private Internets";   February 1996
   [RFC1900]B. Carpenter, Y. Rekhter, "Renumbering Needs Work";
    February 1996
   [RPS]Work in  progerss,  C. Alaettinoglu, T. Bates, E. Gerich, M. Terpstra,
     C. Villamizer, "Routing Policy Specification Language,"
   [RFC1812] F. Baker, "Requirements for IP Version 4 Routers", 06/22/1995.
   [Rigney]  "Remote Authentication Dial In User Service (RADIUS)", C. Rigney,
     A. Rubens, W. Simpson,, 07/23/1996, <draft-ietf-radius-radius-05.txt>
   [Carpenter]  Message to PIER Mailing List, see PIER Archives
   [Lear]  Message to PIER Mailing List, see PIER Archives
   [deGroot]   Message to PIER Mailing List, see PIER Archives
   [Wobus] "DHCP FAQ Memo,"  http://web.syr.edu/~jmwobus/comfaqs/dhcp.faq.html

18.  Author's Address

   Howard C. Berkowitz
   PSC International
   1600 Spring Hill Road, Suite 310
   Vienna VA 22182

   phone: +1 703 998 5819
   email: hcb@clark.net

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