EIGRP: Auto Summarization

Answered Question
Dec 31st, 2008

Everything I read says that EIGRP summarizes on classful boundaries. When I look at the routing table (with auto-summary turned on), I see classless routes (ex. 10.210.0.0).

Can someone explain this to me why this is so. Thanks.

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Also, if I implement "no auto-summary", what advantage will I have by utilizing the "ip summary-address eigrp" command under an interface, other than minimizing the routing table (and perhaps memory usage).

Thanks

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Correct Answer by Jon Marshall about 7 years 11 months ago

Dwayne

Auto-summary occurs across major networks ie.

(172.16.10.1/24) R1 (172.16.11.1/24) ->(172.16.11.2/24) R2 (192.168.5.1/24) ->(192.168.5.2/24) R3 (192.168.6.1/24) ->(192.168.6.2) R4 (172.16.12.1/24)

You have auto-summary turned on for R2 & R4.

In R1's routing table there will still be an entries for 172.16.10.0/24 and 172.16.11.0/24. And the same for R2.

But because R2 connects to R3 on an interface in the 192.168.5.0 network and it has auto-summary turned on it will advertise 172.16.0.0 to R3.

R4 is in the same postion. Because it has an interface in the 192,168.6.0 network and it has auto-summary turned on it advertises 172.16.0.0 to R3.

This is a problem because R3 is now receiving 2 routes for 172.16.0.0 goiing different ways.

The solution is to turn off auto-summary on either R2 or R4 so that the more specific routes are received from one of the routers. In reality you generally turn off auto-summary on both routers unless you need it.

The "ip summary-address eigrp" is used for something quite different. This is used to summarise a group of subnets within the same major network but not necessarily across major network boundaries.

The advantage of using it is as you say you minimize the routing table but also when you summarise with this command all more specific routes are suppressed. So if a change occurs in the network and the detail changes for one of the more specific routes it will not be propogated past the "ip summary-address eigrp". This is what the EIGRP stub network concept is for.

Jon

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Correct Answer
Jon Marshall Wed, 12/31/2008 - 21:15

Dwayne

Auto-summary occurs across major networks ie.

(172.16.10.1/24) R1 (172.16.11.1/24) ->(172.16.11.2/24) R2 (192.168.5.1/24) ->(192.168.5.2/24) R3 (192.168.6.1/24) ->(192.168.6.2) R4 (172.16.12.1/24)

You have auto-summary turned on for R2 & R4.

In R1's routing table there will still be an entries for 172.16.10.0/24 and 172.16.11.0/24. And the same for R2.

But because R2 connects to R3 on an interface in the 192.168.5.0 network and it has auto-summary turned on it will advertise 172.16.0.0 to R3.

R4 is in the same postion. Because it has an interface in the 192,168.6.0 network and it has auto-summary turned on it advertises 172.16.0.0 to R3.

This is a problem because R3 is now receiving 2 routes for 172.16.0.0 goiing different ways.

The solution is to turn off auto-summary on either R2 or R4 so that the more specific routes are received from one of the routers. In reality you generally turn off auto-summary on both routers unless you need it.

The "ip summary-address eigrp" is used for something quite different. This is used to summarise a group of subnets within the same major network but not necessarily across major network boundaries.

The advantage of using it is as you say you minimize the routing table but also when you summarise with this command all more specific routes are suppressed. So if a change occurs in the network and the detail changes for one of the more specific routes it will not be propogated past the "ip summary-address eigrp". This is what the EIGRP stub network concept is for.

Jon

dphills18 Thu, 01/01/2009 - 06:51

But see, in my situation, where I have auto-summary turned on, I am getting none classful summaries.

With the example you gave, R3 would be receiving a route of 172.16.11.0, instead of 172.16.0.0.

It is confusing because we are using class a networks, however, I am seeing routes like 10.210.0.0 in my routing table, even though I have auto-summary turned on on both devices.

Richard Burts Thu, 01/01/2009 - 11:21

Dwayne

You have not given us enough detail to understand your network and explain to you why you are seeing those route entries. From what you describe I would assume that on one of the routers there is an interface that falls into subnet 10.210.0.0/16.

Perhaps you did not fully understand Jon's explanation that auto summary only works when you cross a major network boundary. The entry that you ask about is in major network 10.0.0.0. That subnet will be advertised in its detail as long as it is going over interfaces in network 10.0.0.0. It would only summarize if it went through an interface in some different network. And from what you have said so far I would guess that the interfaces connecting the routers are in 10.0.0.0.

If you provide some additional detail (such as the output of show ip interface brief and the configuration of eigrp) then we would be able to give better answers.

HTH

Rick

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