RIPv2 Auto-Summary question

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Aug 18th, 2010
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Hi,


I've got an issue in a production environment running RIPv2 at a customers site which I can re-produce in the lab.


RIPv2 with no auto-summary command and serveral 10.x.x.x networks.


My problem is that even after entering "no auto-summary" the rip database still shows a 10.0.0.0/8 auto-summary, now my question is will this actually cause me problems when routing to other networks that fall within 10.0.0.0/8 or is this some form of default fuctionality?


If I'm being too vague please let me know.


Thanks,

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Calin Chiorean Wed, 08/18/2010 - 03:07
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Can you copy / paste here the output of "show ip route rip" ?


At least the part where 10.x.x.x/8 appear...


And one more question, did you enter the networks and then apply "no auto-summary"?


Calin

DuncanM2008 Wed, 08/18/2010 - 03:39
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Hi Calin,


In the case of my lab environment there's no mention of 10.0.0.0/8 in the routing table.


There's also no rip routes at the moment as the interfaces are directly connected (loopback in this case for testing).


As for the question of order of input, I put the network statements in prior to disabling auto-summary.


It's also worth noting that the production environment which I've inherited was configured from the start with auto-summary enabled so when I issued no auto-summary in the production environment it would of been after the network statements had been entered.


If that's any help?

Calin Chiorean Wed, 08/18/2010 - 05:00
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Hello Duncan, Peter


I've asked for the output of "show ip route rip" due to the fact that in the initial post I didn't saw any mention if the summary is present in the routing table or not.

And the second question, was due to the fact the RIPv2 has the behavior that if you have the network statement in place, and only after this you apply "no auto-summary", it seems that the summarized network is still in place in the routing table (somehow like no auto-summary command is not working).

It happened to me that I had to clear ip routing table to have RIPv2 remove the summarized network.


And yes, I've wait the so that RIP timers expire, to have RIP remove the summarized route, but it was still there. Maybe it's just under Dynamips this behavior (as I use this for my preparation) but it might happen.


Also I read on INE forum, that they recommend applying always in the beginning of RIPv2 configuration the command "no auto-summary" to avoid the above behavior.


Second, Peter's answer is correct. If the summary is only in the rip database and not in the routing table, then you have nothing to worry about.


You have here a short screencast. It's not yet finished with annotations but you can check it.


http://www.youtube.com/watch?v=RXt2DgWSH2c


Calin

Peter Paluch Wed, 08/18/2010 - 06:42
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Hi Calin,


And the second question, was due to the fact the RIPv2 has the behavior
that if you have the network statement in place, and only after this
you apply "no auto-summary", it seems that the summarized network is
still in place in the routing table (somehow like no auto-summary
command is not working).


Well, I have not ecountered such a bug before though I am aware of multiple bugs in Cisco's RIP implementation. Still, my few thoughts: the automatic summarization in RIPv2 is performed exclusively when sending routing information, never when receiving it. Thus, deactivating the automatic summarization on one router will have an effect on other routers, not on the routing table of that router itself.


Second, when the automatic summarization is deactivated, Cisco's RIP implementation does not care about sending poisoned updates about the previously advertised major networks. Merely, the router simply stops advertising the summary route and just starts advertising the unsummarized component routes. Thus, it will take the flush_timer seconds for that summary route to be removed from the routing table on a neighboring router. The flush_timer is set to 240 seconds by default (4 minutes) since the last update containing the network.


I suggest having a look at the debug ip rip output to see whether the router still advertises the summary network. I believe that the summary network will cease to be advertised immediately after deactivating the automatic summarization.


This also leads me to the question whether you have indeed waited for at least 4 minutes to see the summary route expire on the second router. Please note that the Dynamips has nothing to do with it - it is merely a virtual machine. Just as VMWare or VirtualBox is not responsible for bugs in Linux or Windows running inside it, the Dynamips is not responsible for possible bugs or incorrect behavior of IOS running in Dynamips.


Best regards,

Peter

Peter Paluch Wed, 08/18/2010 - 03:42
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Duncan, Calin,


The RIP database will always contain automatic summaries despite deactivating the automatic summarization. They are simply precomputed but they will not be advertised.


Best regards,

Peter

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