January 2002
Beginner
238 pages
6h 20m
English
Content preview from IP Routing
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Route Summarization
The default behavior of EIGRP is to summarize on network-number boundaries. This is similar to RIP and IGRP and is a prudent way for a routing protocol to reduce the number of routes that are propagated between routers. However, there are some enhancements in the way EIGRP summarizes routes that merit a closer look.
Automatic Summarization
Say TraderMary’s network expands again, this time with a node in Shannon. Shannon gets connected to the London office via a 56-kbps link, as shown in Figure 4-9.
Figure 4-9. Route summarization
Shannon has three Ethernet segments with an IP subnet on each:
172.20.100.0/24,
172.20.101.0/24, and
172.20.102.0/24. The routers in London and Shannon
are configured to run EIGRP 10 in keeping with the routing protocol
in use in the U.S. Shannon will advertise
172.20.0.0/16 to London
because the serial link from London to
Shannon represents a network-number boundary
(172.20.0.0/172.16.0.0).
Shannon itself will see all
172.16.0.0 subnets (without summarization) because
it has a directly connected 172.16.0.0 network.
In EIGRP, the router doing the summarization will build a route to null0 (line 18) for the summarized address. Let’s check Shannon’s routing table:
Shannon#sh ip route 172.20.0.0
...
172.20.0.0/16 is subnetted, 6 subnets
C 172.20.100.0/24 is directly connected, Ethernet0
C 172.20.101.0/24 is directly connected, Ethernet1
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