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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2 Responses to “Automatic summarization in RIP and EIGRP”

  1. save to my Bookmarks :)

  2. Mohamed Kamal Says:

    Dear Wael,

    Sorry if the question isn’t related to EIGRP summerization. But it’s, generally speaking, about EIGRP.

    I know that we can advertise default routes into an EIGRP domain by any of the following:

    1- Static default router and redistributes it under the EIGRP process.
    2- Using the IP default-network.

    I’ve tried the first option and it worked, everything is alright regardless of the AD of 170! But when I tried the IP default-network command, I reached the following:

    R1#sh run

    interface Loopback0
    ip address
    interface Serial0/0
    ip address
    encapsulation frame-relay
    frame-relay interface-dlci 102
    frame-relay interface-dlci 103
    interface Serial0/1
    ip address
    router eigrp 1
    no auto-summary
    ip http server
    ip classless
    ip default-network

    R1#sh ip route

    Gateway of last resort is not set is subnetted, 1 subnets
    C is directly connected, Loopback0 is subnetted, 1 subnets
    D [90/2297856] via, 00:26:41, Serial0/0 is subnetted, 1 subnets
    D [90/2297856] via, 00:26:41, Serial0/0 is subnetted, 1 subnets
    D [90/2300416] via, 00:26:41, Serial0/0
    [90/2300416] via, 00:26:41, Serial0/0 is subnetted, 1 subnets
    D [90/2297856] via, 00:00:52, Serial0/1 is subnetted, 2 subnets
    D [90/2172416] via, 00:26:41, Serial0/0
    [90/2172416] via, 00:26:41, Serial0/0
    C is directly connected, Serial0/0
    * is subnetted, 1 subnets
    C is directly connected, Serial0/1

    Notice that there is no “ip route” statement created and also there is no route marked as candidate default!

    So what is the problem?

    - will I have to adjust the prefix of the network at the interface to be /8 for the EIGRP to know about and then use it as a default network? .. I guess not!

    - will I have to adjust the network command under the EIGRP to include the /8 prefix network instead of /24 prefix and then EIGRP will now know about the network and use it with ip default-network? .. I guess not!

    NOTE: I hope that you can write a post about IGPs and Candidate Default Routes! especially using the ip default-network as they are tend to be confusing.

    Thanks in advance.

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