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- RIPv1 and IGRP perform autosummarization by default, and cannot be disabled – it is a feature
of classful routing protocols. For RIPv2 and EIGRP, the autosummarization feature which
enabled by default can be disabled with the no auto-summary router subcommand.
Autosummarization allows RIPv2 and EIGRP to be backward-compatible with their predecessors
– RIPv1 and IGRP.
- OSPF and IS-IS do no perform autosummarization and do not even have this feature.
172.16.2.0
Figure 16-1: Autosummarization
- Below shows the routing table on RT1 with just a single route to network 10.0.0.0/8 due to the
autosummarization feature of RT2 (RIPv1).
RT1#sh ip route
Gateway of last resort is not set
RT1
172.16.3.0
S0
RT2
10.3.4.0
10.3.5.0
S1
10.0.0.0 / 8
10.3.6.0
10.3.7.0
172.16.0.0/24 is subnetted, 2 subnets
C 172.16.2.0 is directly connected, Ethernet0
C 172.16.3.0 is directly connected, Serial1
R 10.0.0.0/8 [120/1] via 172.16.3.2, 00:00:15, Serial1
RT1#
RT1#debug ip rip
RIP protocol debugging is on
RT1#
00:08:54: RIP: received v1 update from 172.16.3.2 on Serial1
00:08:54: 10.0.0.0 in 1 hops
RT1#
10.2.1.0
10.2.2.0
10.2.3.0
10.2.4.0
RT1
172.16.1.0
RT2
172.16.3.0
RT3
S0
S0
S0
S1
10.0.0.0 / 8
172.16.2.0
Figure 16-2: The Pitfall of Autosummarization
118
Which route to 10.0.0.0/8 ?!?
10.0.0.0 / 8
10.3.4.0
10.3.5.0
10.3.6.0
10.3.7.0
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