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EIGRP Summarization

Posted on 2007-04-03
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Last Modified: 2012-06-27
Gurus!

I'd like to know more about EIGRP route summarization. When should it be used? When should it not be used? What is the benefit of using it? At the moment, we use auto-summary for every router we configure.

For example, in our environment, we have a 3750G switch/router with 2 VLANs configured:
-VLAN 2 (10.10.5.0 255.255.255.0)
-VLAN 3 (10.10.6.0 255.255.255.0)

I've enabled routing on this device and enabled EIGRP. For the "network" command, should I use:
Switch(config) # network 10.0.0.0
OR
Switch(config)# network 10.10.5.0
AND
Switch(config)# network 10.10.6.0

?
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Question by:COE-IT
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7 Comments
 
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Glowingdark earned 750 total points
ID: 18848404
Hello,
  EIGRP summarizes at the classful boundry (so in your case, it summarizes at 10.0.0.0/8). Typically, you turn off route summarization when you have discontiguous subnets across multiple routers.  If you have routers RA & RB, and RA has interfaces for 10.1.0.0/16, and RB has 10.2.0.0/16, you would want to turn auto-summarization off, because otherwise both routers will advertise as having a route to 10.0.0.0/8 and will ignore the other router's advertisement.  If RA had 10.1.0.0/16 and RB had 192.168.1.0/24, auto summarization would be fine.

Kevin

 
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by:COE-IT
ID: 18850910
So what is the difference between using the commands:

Router(config)#router eigrp 1
Router(config)#network 10.10.5.0
Router(config)#network 10.10.6.0

Versus, using the commands:

Router(config)#router eigrp 1
Router(config)#network 10.0.0.0

if I have these two networks (10.10.5.0 /24 and 10.10.6.0 /24) directly connected to my router?
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Expert Comment

by:Glowingdark
ID: 18852775
Your network statements should use the network addresses for the interfaces you want to participate in EIGRP.  I am not sure if your second example would even work, because no interfaces participate in the 10.0.0.0 network.  I have never actually tried doing that though, so I cannot say for sure.   It isn't 100% correct to look at it this way, but the network statement is kind of like telling the router:  "Make the interface with an IP address that is part of THIS (the network address) network advertise and participate in EIGRP".

If you have many routers with subnets that would overlap at the classfull boundry (/8, /16, or /24), you want to turn off auto-summarization.
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Author Comment

by:COE-IT
ID: 18853000
Glowingdark:

I agree. The only way I've known to configure the network statements was to use specific networks (10.1.5.0) to advertise instead of a classful boundry (10.0.0.0). But, all of our routers use the 10.0.0.0 network statement and it works fine. Could it be that the 10.0.0.0 network statement is a way to summarize all of the 10.x.x.x. networks on the router using a single statement?
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Expert Comment

by:Glowingdark
ID: 18853250
I don't work with EIGRP on a daily basis, so I cannot say for sure what is going on.  You have made me curious enough though that I will probably set up a test environment with this setup to see what is really going on with EIGRP.

What do your routing tables look like?  Do they actually show routes learned from other routers via EIGRP?  Especially those that would be ignored if auto-summarization worked as expected?  I would also be curious of the output from some of the debug commands.

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Author Comment

by:COE-IT
ID: 18853535
My routing tables do have routes learned from other routers via EIGRP. So, using a network statement of 10.0.0.0 does work.

My guess is that using the network statement 10.0.0.0 tells the router to advertise any network which sits on that router which qualifies as a 10.x.x.x network. Obviously, my routers aren't telling each other they only have the 10.0.0.0 network attached.
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Author Comment

by:COE-IT
ID: 18853544
Glowingdark:

I'd be interested to know what you find out from your EIGRP lab.
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