Automatic summarization in RIP and EIGRP

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 is variably subnetted, 2 subnets, 2 masks
D is a summary, 00:04:54, Null0 <-- discard route is variably subnetted, 4 subnets, 2 masks
D [90/2297856] via, 00:06:00, Serial1/0 <-- was not summarized
D is a summary, 00:05:15, Null0 <-- discard route

Router2 routing table:

R2(config-router)#do sh ip route eigrp
D [90/2297856] via, 00:05:29, Serial1/0 <-- Summarized subnet is subnetted, 3 subnets
D [90/2297856] via, 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 to R2.
  • R1 sends un-summarized because its part of the major subnet of the its exit interface.

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