Introduction to Route Summarization

This topic is to discuss the following lesson:


I think there is a mistake in the second line in


It should be “network” instead right ?

Hi Rene

I was thinking in order to fix this sub-optimal routing problem of Router R2, why not use the same summarization for the Gi0/2 on R1?
Please let me know if this would be a solution to this problem.


Your solution would work. What you propose doing would be essentially to suppress all specific /24 routes, and only advertise the /16 summary both to R3 and R2. In this case, since the prefix lengths are the same, R2 would fall back using the lowest metric to get to the summary, which for RIP would simply be hop count. R2 would choose the direct connection to R1.

You are correct. I will request that this get corrected. Thanks for point this out.

Hii Rene,

1st query:
no auto-summary command is issued to disable summary.
and to enable summary can we use R1(config-router)#auto-summary command instead of using R1(config-if)#ip summary-address rip

2nd Query:

RIP: sending v2 update to via GigabitEthernet0/1 (
RIP: build update entries via, metric 1, tag 0 via, metric 1, tag 0 via, metric 1, tag 0 via, metric 1, tag 0 

In above RIP update why network is advertising via"" ?


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!


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!

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!


Thanks for you reply… yet… that was not the problem… the specific routers still there in R1. I did some other tests… when I execute the “no ip summary-command” my router R1 sends a poison metric of 16 with the summary addres… so when I type again in R1 “ip summary-address” to activate again the summary address… R2 is sending me back the poison reverse message telling me the summary address is with metric of 16. So I’m assuming router R1 listens to this poison reverse message and does not configure the summary address anymore… and as I DID configure the sumarry address the router R1 is not sending the specific routers to R2… not sure if my logic sounds ok…
Another question… when is poison reverse message usefull?.. I dont understand it…because when a router “Rx” send a poison route… it sends it to all his neighboring routers… so there is no way any other router could send the same “route poisoned” back to “Rx”