PIER Working Group H. Berkowitz
INTERNET DRAFT PSC International
August 1996
Expires in six months
Router Renumbering Guide
draft-ietf-pier-rr-02.txt
Status of this Memo
This document is an Internet Draft. Internet Drafts are working
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Abstract
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.
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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
reasons.
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
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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.
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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
subnet.
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
backbone.
Global Internet
^
|
|
Back1-------------------Back2
| |
+-----------+ +----------+
| | | |
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
switches).
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
renumbering.
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
address.
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 10.1.0.1/16 in stub
domain A to 10.1.0.1/16 in stub domain B. In this case, both
10.1.0.1 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
permit.
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
environment.
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.
6.1.1.1 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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6.1.1.2 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 10.0.0.0/8 as its primary prefix, but
has introduced an ISP whose registered addresses were in
172.31.0.0/16.
Assume a RIPv1 system of three routers:
10.1.0.1/16 10.2.0.1/16
| |
| |
+-------------------------------------+
| Router 1 |
+-------------------------------------+
| 172.31.1.1/24
|
|
| 172.31.1.2/24
+-------------------------------------+
| Router 2 |<------OUTSIDE
+-------------------------------------+
| 172.31.2.1/24
|
|
| 172.31.2.2/24
+-------------------------------------+
| Router 3 |
+-------------------------------------+
| |
| |
10.3.0.1/16 10.4.0.1/16
Router 1 can reach its two locally connected subnets, 10.1.0.0/16
and 10.2.0.0/16. It will aggregate them into a single announcement
of 10.0.0.0/8 when it advertises out the 172.31.1.1 interface.
In like manner, Router 3 can reach its two locally connected subnets,
10.3.0.0/16 and 10.4.0.0/16. It will aggregate them into a single
announcement of 10.0.0.0/8 when it advertises out the 172.31.2.2
interface.
When Router 2 receives a packet from its "outside" interface destined,
say, to 10.1.1.56/16, where does it send it? Router 2 has received
two advertisements of 10.0.0.0 on different interfaces, without any
detail of subnets inside 10.0.0.0. Router 2 has an ambiguous routing
table in terms of the next hop to a subnet of 10.0.0.0. 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 10.0.0.0.
Connectivity, however, also will break between Routers 1 and 3,
because Router 2 does not know the next hop for any subnet of
10.0.0.0.
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 10.0.0.0. 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
10.0.0.0.
Another way to deal with the discontiguous network problem is to
assign secondary addresses in 10.0.0.0 to the R1-R2 and R2-R3
interfaces, which would allow the 10.0.0.0 subnets to be advertised
to R2. This would work as long as there is no problem in advertising
the 10.0.0.0 subnets into the R2 routing system. There would be a
problem, for example, if the 10.0.0.0 address were in the private
address space but the R2 primary addresses were registered, and
R2 advertised the 10.0.0.0 addresses to the outside.
This problem can be handled if R2 has filtering mechanisms that can
selectively block 10.0.0.0 advertisements to the outside world.
The configuration, however, will become more and more complicated.
6.1.1.3 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 172.31.0.0/16 as its major address, which is in
the Class B format. This is actually subnetted with eight bits of
subnetting -- 172.31.0.0/24. Individual hosts unaware of VLSM,
however, either infer Class B from the address value, or are told
that the subnet mask in effect is 255.255.0.0.
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
renumbering.
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
positions.
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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:
zerobits(position,length):
clear <length> bits starting at <position>
orbits(position,length,newbits)
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],
zerobits(0,PrefixLength[j])
orbits(0,PrefixLength[j],NewPrefix[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,
zerobits(0,highOrderPartLength[j])
orbits(0,highOrderPartLength,newHighOrderPart[k])
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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],
zerobits(highOrderPartLength[j],lowOrderPartLength[j])
orbits(highOrderPartLength[j],
lowOrderPartLength[j],newLowOrderPart[k])
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]
then
zerobits(highOrderPartLength[k],newLowOrderPartLength[k])
orbits(highOrderPartLength[k],newLowOrderPart[k])
end
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
addresses.
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
ftp://ftp.uu.net/published/oreilly/nutshell/dnsbind/dns.tar.Z.
Another tool, makezones, is part of the current BIND distribution,
and can also be obtained from
ftp://ftp.cus.cam.ac.uk/pub/software/programs/DNS/makezones
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,
ARP).
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 10.1.0.0/16 address space is used for
global identifiers. A TCP tunnel runs from 10.1.0.1 to 10.1.0.2, but its
traffic is load-shared among the two real links, 172.31.1.0 and 172.31.2.0.
172.31.4.1/24 172.31.5.1/24
| |
| |
+-------------------------------------+
| Router 1 |
| |
| 10.1.0.1/16 |
| # |
+-------------------#-----------------+
| 172.31.1.1/24 # | 172.31.2.1/24
| # |
| # |
| # |
| # |
| # |
| # |
| 172.31.1.2/24 # | 172.31.2.2/24
+-------------------#-----------------+
| Router 2 |
| |
| 10.1.0.2/16 |
| |
+-------------------------------------+
| |
| |
172.31.5.1/24 172.31.6.1/24
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
ports.
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
addresses.
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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
path.
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
routers.
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 |-------|
| 192.168.1.1 | 10.1.0.1/16| | 10.1.0.2/16| |
| /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 10.1.0.1/16
and R2 has 10.1.0.2/16.
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 10.1.0.1 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
interface.
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
routing.
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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
address.
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,
255.255.255.255). 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
addressing.
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
router.
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
effort.
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
scope.
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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
definitions.
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
implemented.
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
routes.
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
cutover.
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
effort.
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
them.
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.
draft-ietf-pier-rr-02.txt [Page 39]
INTERNET-DRAFT Router Renumbering Guide August 1996
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;
draft-ietf-pier-ovrvw-01.txt
[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)."
[Hubbard] Work in Progress; "INTERNET REGISTRY IP ALLOCATION GUIDELINES",
K. Hubbard, J. Postel, M. Kosters, D. Conrad, D. Karrenberg;
draft-hubbard-registry-guidelines-04.txt
[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,"
draft-ietf-rps-rpsl-xx.txt
[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
draft-ietf-pier-rr-02.txt [Page 40]
