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  1. Hello Chandrasekhar

    The auto-summary command restores the default auto summarization behaviour of the routing protocol. RIP Version 1 always uses automatic summarization. If you are using RIP Version 2, you can turn off automatic summarization by specifying the no auto-summary command. Disable automatic summarization if you must perform routing between disconnected subnets. When automatic summarization is off, subnets are advertised. Note that this is a command implemented generally for the whole router under the RIP configuration.

    On the other hand, the ip summary-address command that you stated indicates to the router how to summarize routes on specific interfaces. Notice that the command is implemented on the interface. You can specify how summarization takes place, in a different manner than the default.

    The address after the “via” in the output shows the best next-hop address. If the field is set to the address of the advertising router is the best next-hop address.

    I hope this has been helpful!


  2. Hello… I have a question… and not sure why my routers are working that way… I have a summary address of 3 networks being advertised from R1 to R2… I used the command ip summary-address rip to publish that route from R1 to a neighbor router R2…ok I desactivate this summary address with the command no ip summary-address rip on router R1, and after a few seconds AGAIN I activate the command ip summary-address rip on router R1, the problem is that now the summary address is gone forever… my neighboring router R2 doest not receive anymore the summary addres nor the other routes that were part of the summary address. I used debug ip rip… and the summary address is NOT being sending to the neighbor router even with the command ip summary-address rip being active in the interface, and the routes to the specific networks also are not being forwarded anymore to neighboring router R2. The only way to R2 to learn again those routes is desactivating router rip and configuring again. Any clue why this is working this way? (I also waited the 240 seconds for any timers to finish…and nothing… no specific routers being forwared to R2 nor the summary-address).


    Thanks in advance!

  3. Hello Alvaro

    That’s interesting. Normally, when you reinstate the summary-address command, the summary address should appear in the RIP neighbor. Take a look at this text from a Cisco Document (link below).

    As long as there are child routes for a summary address, the address remains in the routing database. When the last child route is removed, the summary entry also is removed from the database. This method of handling database entries reduces the number of entries in the database because each child route is not listed in an entry, and the aggregate entry itself is removed when there are no longer any valid child routes for it.

    The only reason that a summary will not appear is if there are no child routes remaining in the local router, that is, there are no routes that fall within that summary. Here is the link from where this text was found. This Cisco document further describes route summarization and may be of help to you.

    What I suggest is to examine, if you remove the summary-address command, do you see the individual routes that fall under that summary being advertised? If not, then this is the reason why the summary is also not advertised. If however they are there, then you should check to see if there is an error anywhere else in the configuration that may be causing this.

    I hope this gives you some help in continuing your troubleshooting.

    I hope this has been helpful!


  4. Hello Alvaro

    Yes Alvaro, that makes sense. Poison reverse will cause R1 not to put the routes in the routing table. However, once you reinstate the summary route, after a certain amount of time, the poison reverse message will have timed out, this is how it should function. During the time that the networks are still “poisoned” can you use show ip protocols and show ip rip database and you can also continue to watch RIP debugs to see if/when the poison updates stop and if/when the networks get reinstated. Once the poison reverse updates expire, the networks should get reinstated. If not, with the tools mentioned, you should be able to determine why not.

    Poison reverse is useful in networks where you may have routing loops. If you had a third router in your topology, then R1 may have removed the summary routes and sent out a route poison update to R2, but in the meantime, R3 may send one of its periodic updates indicating that it has a route to the (no longer available) summary route, and R1 may reinstate it even though it was removed. More information on reverse poisoning can be found in this lesson which explains it in detail.

    I hope this has been helpful!


  5. Okay… thanks for your time. =)

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