Summarization in RIP and EIGRP can be configured manually using the ip summary-address command or automatically using auto-summary command. when auto summarization is in effect routers behave as shown in the steps below:
- Subnets are summarized to their classful boundary before being sent to neighbors if they are not part of the major subnet of the exit interface.
- EIGRP automatically installs a discard route in the local routing table. RIP does not install a discard route by default.
In the following example we will explore EIGRP with automatic summarization enabled:
Router1 is connected to Router2 by a serial link and each router has loopback interfaces configured as shown in following diagram
Router1 Configuration (same configuration applies to Router2)
|R1(config)#router eigrp 10
Router1 routing table:
|R1(config-router)#do sh ip route eigrp
18.104.22.168/16 is variably subnetted, 2 subnets, 2 masks
D 22.214.171.124/16 is a summary, 00:04:54, Null0 <– discard route
126.96.36.199/16 is variably subnetted, 4 subnets, 2 masks
D 188.8.131.52/24 [90/2297856] via 184.108.40.206, 00:06:00, Serial1/0 <– was not summarized
D 220.127.116.11/16 is a summary, 00:05:15, Null0 <– discard route
Router2 routing table:
|R2(config-router)#do sh ip route eigrp
D 18.104.22.168/16 [90/2297856] via 22.214.171.124, 00:05:29, Serial1/0 <– Summarized subnet
126.96.36.199/24 is subnetted, 3 subnets
D 188.8.131.52 [90/2297856] via 184.108.40.206, 00:05:50, Serial1/0 <– non summarized route
Automatic summarization summarized:
- R1 installs discard routes pointing to null interface before sending summaries.
- R1 sends the summarized route 220.127.116.11/16 to R2.
- R1 sends 18.104.22.168/24 un-summarized because its part of the major subnet of the its exit interface.