IP summary-address eigrp Command

Answered Question
Apr 21st, 2012

Hi everybody ,

I'm confused about

IP summary-address eigrp Command when it use on specific interface , when we need to use it and what is the difference between it and between manual summerization for network ex. 172.16.0.0 0.0.3.255

regards,

I have this problem too.
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Correct Answer by Richard Burts about 2 years 3 hours ago

Your understanding of the network command used in EIGRP is not correct. You are assuming that entering

network 10.0.4.0 255.255.252.0

will summarize the advertisement. That is not the case. If you enter that command EIGRP will look for interfaces that match and will advertise the subnets from those interfaces. It will not summarize any entries.

To explain it in a slightly different way: EIGRP uses the network command only to identify what interfaces are to be included in the dynamic routing protocol process. Once EIGRP has included an interface in the routing protocol process it then looks at the subnet configured on the interface and will advertise that subnet to its neighbors.

HTH

Rick

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ryan.wolfe Sat, 04/21/2012 - 09:43

EIGRP auto summarization applies to the entire EIGRP process. The ip summary-address command let's you manually summarize that network only out of the applied interface. You could have a network out that interface that could be summarized but other routes in other places that can't be summarized.

If you manually summarize on the interface you can reduce the size of the routing table for the routers out that interface. The smaller the routing table, the faster the routing occurs.

Of course this would only make a noticeable difference on large networks, but it is still best practice to maintain the most optimal routing possible.

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zaherercisco Sat, 04/21/2012 - 09:50

thanks Ryan,

I'm trying to find scenario -example for this command to make think more clear to me , if anyone have , will highly appreciated.

regards,

ryan.wolfe Sat, 04/21/2012 - 10:33

I drew up a quick scenario.. hope it helps out a bit. Also, something I forgot to mention in my original post is that manual summarization allows you to summarize at places other than the classful boundary. In the example I provided, I summarized at /22 but the 10.0.0.0 is classfully summarized at the /8 boundary.

HTH

zaherercisco Sat, 04/21/2012 - 13:45

thanks deeply ryan , but what if we use the normal  command

network 10.0.0.0 255.255.252.0 instead of ip summary-address  what difference will be ?

regards,

nkarpysh Sat, 04/21/2012 - 18:13

Network command is used not to advertise routes in IGP. It is used to select interfaces which will participate in this IGP process. So you can either select specific interface selecting single ip with network command:

network 10.x.x.x 0.0.0.0

or select multiple interfaces falling in common range by configuring wider network:

10.0.0.0 255.255.252.0

Thus this command only tell which interfaces should participate in IGP. Interfaces in turn will start advertise their networks but not the summary

Nik

zaherercisco Sun, 04/22/2012 - 02:52

sorry guys but there is something I'm missing here ,

as far as I knew since I will configure R4 like follow

eigrp 1

network 10.0.0.0 255.255.252.0

and R5

eigrp 1

network 10.0.4.0 255.255.252.0

this mean I've advertise the summary of both network in clasless manner on all the interfaces that connected to R4 and R5 , so what difference make the ip-summary address do ? :/

Correct Answer
Richard Burts Sun, 04/22/2012 - 03:45

Your understanding of the network command used in EIGRP is not correct. You are assuming that entering

network 10.0.4.0 255.255.252.0

will summarize the advertisement. That is not the case. If you enter that command EIGRP will look for interfaces that match and will advertise the subnets from those interfaces. It will not summarize any entries.

To explain it in a slightly different way: EIGRP uses the network command only to identify what interfaces are to be included in the dynamic routing protocol process. Once EIGRP has included an interface in the routing protocol process it then looks at the subnet configured on the interface and will advertise that subnet to its neighbors.

HTH

Rick

Richard Burts Tue, 04/24/2012 - 08:47

I am glad that our discussion helped you to correctly understand the difference. Thank you for using the rating system to mark this question as answered. It makes the forum more useful when people can read a question and can know that an answer was found. Your marking has contributed to this process.

HTH

Rick

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