[Search] [pdf|bibtex] [Tracker] [WG] [Email] [Nits]

Versions: 00 01                                                         
INTERNET-DRAFT                                John W. Stewart, III / ISI
<draft-ietf-idr-aggregation-tutorial-00.txt>           Enke Chen / Cisco
                                                               July 1997

                        Route Aggregation Tutorial

Status of this Memo

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

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

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


   Route aggregation is critical to the long-term viability of the
   Internet.  This document presents practical information that network
   managers can use to both understand the concepts of aggregation as
   well as put those concepts into use in order to do their part to make
   the Internet stable and allow its continued growth.  The intended
   audience for this document is anyone responsible for configuring a
   network which has its own Autonomous System Number (ASN) and
   exchanges routing information with its Internet Service Provider(s)
   (ISP(s)) using the Border Gateway Protocol (BGP).  This document does
   not cover multi-homing, though multi-homed sites can still benefit
   from understanding this material.

1. Introduction

   The long-term viability of the Internet depends on its ability to
   support the continued growth in demand.  A large part of its ability
   to grow is dependent on the successful scaling of the routing system.
   Because the complexity of the Internet's routing system is a function
   of the number of reachable destinations, great care must be taken
   that, as the net grows, the demands on the routing system don't
   outpace advances in hardware and software.

Stewart & Chen                                                  [Page 1]

INTERNET-DRAFT         Route Aggregation Tutorial              July 1997

   In the early 1990s, the paradigm for large scale Internet routing
   changed from a "Classful" system to a "Classless" system.  The
   Classless system applies techniques of hierarchy to achieve large
   scaling.  In order for Classless routing to achieve its goal of
   allowing the routing system to scale very well, networks in all areas
   of the Internet must be vigilant about "route aggregation."  This
   document provides educational information, both conceptual and
   practical, in an effort to encourage efficient aggregation throughout
   the Internet.

2. Network Classes and the Bit-Level Detail

   As originally specified in the early 1980s, the Internet Protocol
   (IP) included the idea of network "Classes."  [1]  In IP, a certain
   number of bits in the 32-bit addresses refer to the network and the
   remainder of the bits refer to a host on that network.  (In the mid
   1980s IP was extended such that part of the host bits can refer to a
   subnet and the remainder would refer to a host on that subnet.  [2])
   The point of the different Classes was to have addresses with
   different numbers of network/host bits.  The Class of an address
   could be determined by the high-order bits.  A Class A address had
   "0" as the high-order bit, and then 7 bits of network and 24 bits of
   host; a Class B address had "10" as the high-order two bits, and then
   14 bits of network and 16 bits of host; and a Class C address had
   "110" as the high-order three bits and then 21 bits of network and 8
   bits of host.  Looking at an address in "dotted quad notation" (e.g.,, Class A networks have a first number of 0-127, Class B
   networks have a first number of 128-191 and Class C networks have a
   first number of 192-223.  A Class A network could number 1.7 million
   hosts, a Class B 65,000 and a Class C 256.  Diagramatically:

Stewart & Chen                                                  [Page 2]

INTERNET-DRAFT         Route Aggregation Tutorial              July 1997

          3 3 3 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1
          2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1
   Class +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     A   |0                                                              |

          3 3 3 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1
          2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1
   Class +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     B   |1 0                                                            |

          3 3 3 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1
          2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1
   Class +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     C   |1 1 0                                                          |

   Although the original intent of having Classes was to allow for
   flexible addressing, experience showed that the hard boundary of the
   three Classes actually made the addressing less flexible.  For
   example, if a site connecting to the Internet needed to address 300
   hosts, then a Class C network wouldn't be adequate and a Class B
   would need to be assigned.  This resulted in poor utilization of the
   assigned address space and caused a faster-than-necessary rate of
   consumption of the available IP address space.

   Another problem with the scalability of Internet routing under the
   Classful system had to do with the address allocation policies used.
   At that time, when a site connected to the Internet, it would go to a
   central registry to get a unique IP network and then it would go to
   an ISP to procure connectivity.  What this means is that if an ISP
   had 1000 customers, each of whom had been assigned a Classful network
   of some type, then that ISP would have to announce each of those 1000
   networks to other providers in the Internet.  In other words, the
   Internet's routing system was not taking advantage of the inherent
   provider/subscriber hierarchy and instead was being "flat-routed."

3. The Introduction of CIDR

   In the early 1990s, a number of ISPs began to have operational
   problems related to the size of a full Internet routing table because
   of the limited amount of memory available in commercial routers.  (A

Stewart & Chen                                                  [Page 3]

INTERNET-DRAFT         Route Aggregation Tutorial              July 1997

   "full routing table" means a routing table which does not contain a
   default route and instead contains an entry for every active network
   in the Internet.)  Because of these problems, Classless Inter-Domain
   Routing (CIDR) was created.  [3]

   CIDR removed the idea of Classes from IP.  Instead of having networks
   with an implied number of bits referring to network/host, there are
   "prefixes" with an associated mask explicitly identifying which bits
   refer to network/host.  For example, the prefix "" with a
   mask of "" has 24 bits of network and 8 bits of host
   (i.e., it can address the same number of hosts as a Class C network
   even though the prefix is in the Class A range).  The CIDR paradigm
   prefers the term "prefix" over "network" because it's more clear that
   no Class is being implied.  Another way to write this example prefix
   is "", meaning that the mask contains 24 1s in the
   high-order portion of the mask.

   The strength of CIDR is that the masks can be on arbitrary bit
   boundaries and don't have to be on byte boundaries.  So for example,
   going back to the case of the site which needs to address 300 hosts,
   the site could be allocated a "/23" (i.e., a prefix which has 23 bits
   for network and 9 bits for host, thus allowing 512 hosts to be
   addressed with the single prefix).

   To complete the picture, in order for CIDR to actually help achieve
   better scaling of Internet routing, a specific address allocation
   architecture must be used.  [4]  Rather than the pre-CIDR style where
   sites would go to a centralized registry to get an address which does
   not take into account where that site connects to the Internet,
   CIDR-style address allocation involves registries allocating address
   space to ISPs who, in turn, sub-allocate it to their customers.  So
   for example, a registry might allocate the prefix
   (called a "CIDR block") to ISP1, and then ISP1 could sub-allocate to SmallCustomer1, to SmallCustomer2, to MediumCustomer and to
   LargeCustomer.  The benefit, then, is that when ISP1 exchanges
   routing information with other ISPs, it only needs to announce the
   single prefix and not each of the individual prefixes
   used by its customers.  The ability to merge multiple prefixes which
   have some number of leading bits in common is called "aggregation."

   In 1993, the deployment of a routing protocol which supported CIDR
   (specifically BGP Version 4 [5]) had an immediate and measurably
   positive effect on route scaling.  Immediately after its deployment a
   full routing table went down in size in absolute numbers (this was
   possible only because address allocation had already been done for
   some time in the CIDR style even though the routing hadn't yet taken
   advantage of it) and, more importantly, the rate of growth was

Stewart & Chen                                                  [Page 4]

INTERNET-DRAFT         Route Aggregation Tutorial              July 1997


4. A Note on Renumbering

   The crux of CIDR is that the Internet's generally hierarchical
   topology is being reflected in the addressing.  As a result, if a
   site started out as a customer of ISP1 and is thus numbered out of
   one of ISP1's CIDR blocks, but then that site terminates the
   relationship with ISP1 and "rehomes" to ISP2, then the site would
   need to renumber its nodes to be part of one of ISP2's CIDR blocks.
   The major reason for this is to retain efficiency in the routing
   system.  [6]

   Renumbering is an unfortunate necessity in the current IPv4 Internet.
   This is the reason for the recent advance of renumbering technology
   in IPv4 (e.g., DHCP [7]) as well as the focus of easy renumbering in
   IPv6.  [8]  Sites should keep this "unfortunate necessity" in mind
   when deploying equipment to make sure that their infrastructure can
   be renumbered easily if that becomes necessary.

5. Practical Aggregation

   As stated earlier, aggregation refers to the combining of multiple
   contiguous prefixes into a single prefix.  For example, assume the
   prefixes and  The binary
   representation for is:

          3 3 3 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1
          2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1
         |1 1 0 1 0 0 0 1 0 1 1 1 1 0 1 1 0 0 0 0 1 0 1 0 0 0 0 0 0 0 0 0|
         |<---- 209 ---->|<---- 123 ---->|<----  10 ---->|<----  0  ---->|

   And the binary representation for is

          3 3 3 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1
          2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1
         |1 1 0 1 0 0 0 1 0 1 1 1 1 0 1 1 0 0 0 0 1 0 1 1 0 0 0 0 0 0 0 0|
         |<---- 209 ---->|<---- 123 ---->|<----  11 ---->|<----  0  ---->|

   The important thing to note here is that the two networks can be

Stewart & Chen                                                  [Page 5]

INTERNET-DRAFT         Route Aggregation Tutorial              July 1997

   aggregated into the single prefix because they have
   the leading 23 bits in common.

   The example above is very simple.  A real example of a very large
   degree of aggregation is the prefix, which covers the
   4096 Class C networks through  It's
   obvious from this example how profound an impact aggregation can have
   on the size of a routing table and the resources required for the
   associated storage and computation.

   It is important to aggregate as much as possible, even in the simple
   example presented earlier, because small non-optimalities can add up
   and result in a poorly aggregated global routing system.  If you
   exchange routes with your provider using BGP, then it is your
   responsibility to do the aggregation configuration.  (Note that
   aggregation can only be done with BGP4, so if you are running an
   earlier version of BGP, you should upgrade your software; most major
   router manufacturers have implemented BGP4.)  Assuming that your AS
   number is 5555, your provider's AS number is 2222 and the IP address
   of your provider's BGP speaker is, the Cisco syntax for
   configuring the aggregation would be:

      router bgp 5555
       network mask
       neighbor remote-as 2222
      ip route Ethernet0
      ip route Ethernet1
      ip route Null0 254

   The "network" line in the BGP section tells the router to announce
   that network if it has a route to it.  The third static route for creates a "pull-up" route for the aggregate so that
   the router actually has a route to it so that the "network" line
   takes effect and the prefix is announced.  The static route for the
   aggregate is only needed in order for the "network" line to take
   effect; that static route will never be used for packet forwarding
   because the static routes for the individual Class C networks are
   more specific and therefore take precedence.  This configuration
   information is required only on the router which speaks BGP with your
   provider's router.

Stewart & Chen                                                  [Page 6]

INTERNET-DRAFT         Route Aggregation Tutorial              July 1997

6.  References

   [1] Postel, J., "Internet Protocol", RFC 791, September 1991.

   [2] Postel, J., Mogul, J.C., "Internet Standard Subnetting
   Procedure", RFC 950, August 1985.

   [3] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless Inter-
   Domain Routing (CIDR): an Address Assignment and Aggregation
   Strategy", RFC 1519, September 1993.

   [4] Rekhter, Y., Li, T., "An Architecture for IP Address Allocation
   with CIDR", RFC 1518, September 1993.

   [5] Rekhter, Y., and Li, T., "A Border Gateway Protocol 4 (BGP-4)",
   RFC1771, March 1995.

   [6] Ferguson, P., Berkowitz, H., "Network Renumbering Overview:  Why
   would I want it and what is it anyway?", RFC 2071, January 1997.

   [7] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March

   [8] Thomson, S., Narten, T., "IPv6 Stateless Address
   Autoconfiguration", RFC 1971, August 1996.

7.  Authors' Addresses

   John W. Stewart, III
   4350 North Fairfax Drive
   Suite 620
   Arlington, VA  22203
   phone: +1 703 807 0132
   email: jstewart@isi.edu

   Enke Chen
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134-1706
   Phone: +1 408 527 4652
   email: enkechen@cisco.com

Stewart & Chen                                                  [Page 7]