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Eigrp automatic summarization

kpjunglekpjungle Member Posts: 426
in CCNP
Hi all,

Doing some work on EIGRP and automatic summarization. Does a router only summarize its own connected networks when automatic summarization is in place?

R1 <-> R2 <-> R3

If R1 advertises 7.7.7.0/24 to R2, from my labwork R2 wont summarize the route to 7.0.0.0/8 when it sends it to R3, as I would have expected?
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    EdTheLadEdTheLad Member Posts: 2,111 ■■■■□□□□□□
    In a word,YES!
    Networking, sometimes i love it, mostly i hate it.Its all about the $$$$
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    kpjunglekpjungle Member Posts: 426
    EdTheLad wrote: »
    In a word,YES!

    Hehe, nice and simple! thanks!
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    tech-airmantech-airman Member Posts: 953
    kpjungle,
    kpjungle wrote: »
    Hi all,

    Doing some work on EIGRP and automatic summarization. Does a router only summarize its own connected networks when automatic summarization is in place?

    It depends. Is the connected network also a subnet of the classful network?
    kpjungle wrote: »
    R1 <-> R2 <-> R3

    If R1 advertises 7.7.7.0/24 to R2, from my labwork R2 wont summarize the route to 7.0.0.0/8 when it sends it to R3, as I would have expected?

    Do you have automatic summarization enabled on R1?
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    EdTheLadEdTheLad Member Posts: 2,111 ■■■■□□□□□□
    tech-airman wrote: »
    It depends. Is the connected network also a subnet of the classful network?
    It doesn't depend, this is an eigrp caveat, the connected network is not part of the same subnet.
    tech-airman wrote: »
    Do you have automatic summarization enabled on R1?

    If R1 had auto-summary enabled it would not send 7.7.7.0/24 to R2 it would send 7.0.0.0/8, the question is in regards to how R2 handles its summarization.Auto-summary is enabled on R2, R2 has a route 7.7.7.0/24 towards R1, the link between R2 and R3 uses a different classful network than 7.0.0.0, you would expect R2 is classfully summarize 7.7.7.0/24 due to the boundary at R3,but it doesnt because 7.7.7.0/24 is not a local network.
    Networking, sometimes i love it, mostly i hate it.Its all about the $$$$
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    kpjunglekpjungle Member Posts: 426
    EdTheLad wrote: »
    It doesn't depend, this is an eigrp caveat, the connected network is not part of the same subnet.



    If R1 had auto-summary enabled it would not send 7.7.7.0/24 to R2 it would send 7.0.0.0/8, the question is in regards to how R2 handles its summarization.Auto-summary is enabled on R2, R2 has a route 7.7.7.0/24 towards R1, the link between R2 and R3 uses a different classful network than 7.0.0.0, you would expect R2 is classfully summarize 7.7.7.0/24 due to the boundary at R3,but it doesnt because 7.7.7.0/24 is not a local network.

    Yep. That pretty much sums it up.
    Heres another curveball I ran into with RIP (version 2):

    R1 <-> R2

    R1 does not summarize by default, but R2 is set to. R1 advertises its loopback0 of 192.168.1.0/26 to R2. (the link between R1 and R2 is still a different major net). What behavior would you expect R2 to conform to? My first guess was that it would actually summarize to 192.168.1.0/24 and send it back to R1 (split horizon prevents the SAME route going back to R1, but this is not the same route). This is not what occurs. But if you create a loopback interface on R2 in the same major net (192.168.1.0), such as 192.168.1.65/27, it will advertise it back to R1 as 192.168.1.0/24.

    Interesting no? :)
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    kryollakryolla Member Posts: 785
    The router that has multiple interfaces in two different major network is the router that does summarization.

    Enhanced Interior Gateway Routing Protocol - Cisco Systems
    Studying for CCIE and drinking Home Brew
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    kpjunglekpjungle Member Posts: 426
    kryolla wrote: »
    The router that has multiple interfaces in two different major network is the router that does summarization.

    Enhanced Interior Gateway Routing Protocol - Cisco Systems

    True, for EIGRP. RIP behaves differently though. It will summarize even though it has no interfaces. It does not need a connected route of the same major net to summarize when crossing a boundary.
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    kryollakryolla Member Posts: 785
    yeah eigrp does it according to network boundary where rip does it as well but also across different subnets
    Studying for CCIE and drinking Home Brew
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