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Route Summarization for EIGRP
Three individual route types can be found with OSPF and its various con-
figured areas. The route types are as follows:
Intra-area routes (Type O) Routes that are explicit network or subnet
routes. These must be carried inside a configured area, and all area mem-
ber routers must know about them.
Intra-area routes (Type IA) Routes that exist in the internal autono-
mous system but not in the router's configured area.
External routes (Types E1 and E2) Routes that exchange routing
information between autonomous systems.
Configured areas help divide shared routing information. Area Border
Routers (ABR) advertise IA routers from one area to another area.
Route Summarization for EIGRP
IGRP does not build the same hierarchy tables that OSPF does but is
capable of reducing the learned routes. By default, EIGRP automatically
summarizes its routes when Variable-Length Subnet Masking (VLSM) is not
used. This means that if your addressing scheme uses an 8-, 16-, or 24-bit
mask with a class A, B, or C network, EIGRP will handle route summariza-
tion just fine. If you do use VLSM, then you need to disable the default sum-
marization by using the no auto summary command in the Router
Configuration mode. Then you can manually configure a summarized route
using the ip summary eigrp command on each interface. The command
and syntaxes are as follows:
ip summary-address eigrp <AS number> <network> <mask>
Let's look at an example of the command where the IP address of is connected to another a router that is connected to two other
routers with a network range of to If we write out
the network numbers in bits, we see that the first 20 bits are identical in each
network address. These first 20 bits are referred to as a CIDR Block. This
block allows the network to be advertised as a single route to the outside
world. Instead of keeping a giant routing table of all the networks individu-
ally, the tables have only one entry for all the networks contained in the
Copyright ©2001 SYBEX , Inc., Alameda, CA