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”


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!


Thanks for your reply… and yeah… I did read that lesson thats why I have the question… If a router X, have a route no longer valid, lets say to its LAN segment, it will start sending the poison route to every other interface (to every other RIP router it has directly connected)… so how it is possible that other router could send back the same route that router X is poisoning… I just dont see it how it is possible that RX “reinstate” that route from other router… router X poisoned the route and send it to ALL every neighbor not just to one.

Hello Alvaro

Your logic is sound, and what you are saying makes sense. Even so, practically speaking, poison revers aids in improving convergence speed. According to the RFC 2453 for RIPv2:

In a static sense, advertising reverse routes with a metric of 16
provides no additional information. If there are many routers on one
broadcast network, these extra entries can use significant bandwidth.
The reason they are there is to improve dynamic behavior. When
topology changes, mentioning routes that should not go through the
router as well as those that should can speed up convergence.

I hope this has been helpful!


Well yeah… I was just trying to make sense what you guys put in the lesson… “to avoid loops”… do you guys have a link to further explain this?.. thanks in advance!

Hello Alvaro

I understand your concern. I will let Rene know and see if we can have that explained in more detail in the lesson. Beyond that reference in the lesson, poison reverse is not further described in any lessons for the time being.

Thanks for your feedback!


Okay… thanks for your time. =)

1 Like


Can we able to summarize only static routes and connected routes to a dynamic routing protocol?
Can we summarize dynamic learning routes to another dynamic routing protocol?
Should we use redistribution with summarization to summarize from one dynamic routing protocol to another?


Hello Murat

You can summarize static routes, connected routes, and dynamic routes to a dynamic routing protocol.

Yes, you can do that. Take a look at these lessons:

Yes, that also is something that can and should be done to increase efficiency. Take a look at this link that includes more detailed information about that.

I hope this has been helpful!


You said " The summary route also has a single metric."
What does it mean . Can you elaborate

Hello Sims

What Rene means by this statement is that before the implementation of the summary route, all of the routes represented by that summary will be in the routing table, each with a different path to reach the destination in each case. Each of these different paths correspond to a different metric based on the cost of each particular route.

When a summary route is implemented, all of these routes are represented by that single summary route and all of them will have a single metric and a single path, even if that metric is larger than it was for some routes before the summary route was implemented. The result is sub-optimal routing.

I hope this has been helpful!