2.5 Route Summarization Overview
2.5.4 Route summarization implementation considerations
Route summarization reduces memory use on routers, CPU for recalculations and routing-protocol network traffic. Requirements for summarization to work correctly are as follows:
  • Multiple IP addresses must share the same high-order bits.
  • Routing tables and protocols must base their routing decisions on a 32-bit IP address and prefix length that can be up to 32 bits.
  • Routing protocols must carry the prefix length (subnet mask) with the 32-bit IP address.

For example, consider the binary equivalent of 172.21.136.0/16 and 172.21.140.0/16, as follows:

10101100.00010101.10001000.00000000

10101100.00010101.10001100.00000000

You can count the common high order bits at 21 and then summarize these routes as 172.21.136.0/21. The first 21 bits are common between the two addresses. The number "21" is used as the prefix.

Route Summarization Operation in Cisco Routers

This section discusses the generalities of how Cisco routers handle route summarization. Details about how route summarization operates with a specific IP routing protocol are discussed in the specific protocol chapter.

Cisco routers manage route summarization in two ways:

  • Sending route summaries --- Routing information advertised out an interface is automatically summarized at major (classful) network address boundaries only for some protocols, such as IGRP and RIP. A classful network address is a traditional address that has defined standard boundaries. For example, the address 12.0.0.0 is a classful address using the first byte for the network portion (12.0.0.0/8). Specifically, this automatic summarization occurs for those routes whose classful network address differs from the major network address of the interface to which the advertisement is being sent. For protocols such as OSPF, you must configure summarization.

Route summarization is not always a good solution. You would not want to use route summarization if you needed to advertise all networks across a boundary, such as when you have discontiguous networks (discussed next in this section). Protocols such as EIGRP and RIP2 allow you to disable automatic summarization.

  • Selecting routes from route summaries --- If more than one entry in the routing table matches a particular destination, the longest prefix match in the routing table is used. Several routes might match one destination, but the longest matching prefix is used.

For example, if a routing table has different paths to 172.168.0.0/16 and to 172.168.5.0/24, packets addressed to 172.168.5.99 would be routed through 172.168.5.0/24 because that address has the longest prefix match.

Administrative Distances

The term metric has already been explained as describing the overall desirability of a route to a remote (not locally attached) network.

With Cisco routers, there is another concept called administrative distance, which is a subjective analysis of the believability of a routing protocol. In other words, if a router running two different routing protocols learns about the same network from both protocols, the protocol that has the lower distance associated with it will be the one whose entry is installed in the router's routing table. The types of distances used in the next scenario, which is on running multiple routing protocols concurrently, are as follows:

  • Connected interface Distance 0
  • A subnet that is assigned to an interface in the router takes precedence over an advertised route for the same subnet. If RouterB advertised a route to 168.71.5.0, RouterA would ignore it because it has a connected route to this subnet already.
  • Static route Distance 1
  • A route that has been manually configured into a router. If RouterA had a static route indicating that 168.71.8.0 was reachable via the next hop of 168.71.6.2 (RouterB), this route would override any dynamic route RouterA received from RouterC, even though the RouterA-to- RouterC path is actually shorter in terms of hops.
  • IGRP Distance 100
  • Cisco's proprietary Interior Gateway Routing Protocol.
  • RIP Distance 120
  • An IETF standard routing protocol.

There are several other types of routing protocols and associated distances. When using a combination of routing protocols, keep in mind that the default distance values may cause a router to use a path that you did not intend it to use.

Summarizing Routes in a Discontiguous Network

Classful routing protocols summarize automatically at network boundaries. This behavior, which cannot be changed with RIP and IGRP, has the following important results:

  • Subnets are not advertised to a different major network.
  • Discontiguous subnets are not visible to each other.

In the main figure, the 172.16.5.0/24 and 172.16.6.0/24 subnetworks are not advertised by RIP because RIP cannot advertise subnets. Therefore, each router advertises 172.16.0.0, which leads to confusion when routing across network 172.168.14.0 because this network receives routes about 172.16.0.0 from two different directions, so it cannot make a correct routing decision.

This situation is resolved by RIP2, OSPF, and Enhanced IGRP when summarization is not used because the routes could be advertised with their actual subnet masks.

Advertisements are configurable with OSPF and Enhanced IGRP. Cisco IOS software also provides an IP unnumbered feature that permits noncontiguous (or nonsequential) subnets separated by an unnumbered link. An unnumbered link does not have an address assigned to it.

Note RIP1 and IGRP do not advertise subnets or support noncontiguous subnets.