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Deep dive into EIGRP Summarization

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Introduction

 

We all know the concept of summarization and its benefit however, what we are going to see in this document is something which has not been explained in the books and is derived from knowledge and experience only. Before we start, just to refresh our memories summarization is a method of reducing number of routes in the routing table of a layer 3 device which can be a router or a Multilayer switch. Here I am listing the benefits summarization offers:

1.      Summarization help minimize latency in a complex network, especially when number of devices involved is high.

2.      Because of reduced number of routing table entries, the overhead for routing protocol decreases.

3.      Improved network stability by reducing or eliminating unnecessary routing updates after part of our network undergoes any changes.

4.      Summarization also reduces processor workload and memory requirements on the device.

 

The above benefits are really generic to summarization and apply to all the routing protocols capable of performing summarization, be it EIGRP, OSPF or others.

The only difference while using summarization in EIGRP is that it allows summarization to be done at any point in the network.

 

 

Prerequisites

We will be discussing summarization here assuming that the reader has  basic understanding of EIGRP and knows how to configure it, we are not  going to configure EIGRP here.

 

Components Used

Cisco 3700 series router and IOS 12.3(24a) are used in this configuration example however, this is a protocol related information and is platform independent.

 

 

Network Diagram

EIGRP Summarization1.jpg

Verify

Here is the result from show ip route command at all routers in default configuration (auto-summary enabled)

 

From R3:

 

     172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks

C       172.16.4.0/24 is directly connected, Loopback4

D       172.16.0.0/16 is a summary, 00:02:04, Null0

C       172.16.1.0/24 is directly connected, Loopback1

C       172.16.2.0/24 is directly connected, Loopback2

C       172.16.3.0/24 is directly connected, Loopback3

     10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks

C       10.1.13.0/24 is directly connected, Serial1/0

D       10.1.12.0/24 [90/2681856] via 10.1.13.1, 00:02:15, Serial1/0

D       10.0.0.0/8 is a summary, 00:02:04, Null0

 

From R1:

 

D   172.16.0.0/16 [90/2297856] via 10.1.13.3, 00:02:01, Serial1/1

     10.0.0.0/24 is subnetted, 2 subnets

C       10.1.13.0 is directly connected, Serial1/1

C       10.1.12.0 is directly connected, Serial1/0

 

We can see that R1 is getting one classful network from R3 for 172.16.0.0 network, which is as expected.

 

From R2:

 

D   172.16.0.0/16 [90/2809856] via 10.1.12.1, 00:02:34, Serial1/0

     10.0.0.0/24 is subnetted, 2 subnets

D       10.1.13.0 [90/2681856] via 10.1.12.1, 00:03:12, Serial1/0

C       10.1.12.0 is directly connected, Serial1/0

 

It has got to know about 172.16.0.0 network and 10.1.13.0 network from R1, nothing great right!

Now, let’s get ready for the concept for which the document has been prepared.

Once I disable auto-summary on R3 by using no auto-summary command, here is what we get:

 

Routing table at R3 remains same, nothing unusual:

 

     172.16.0.0/24 is subnetted, 4 subnets

C       172.16.4.0 is directly connected, Loopback4

C       172.16.1.0 is directly connected, Loopback1

C       172.16.2.0 is directly connected, Loopback2

C       172.16.3.0 is directly connected, Loopback3

     10.0.0.0/24 is subnetted, 2 subnets

C       10.1.13.0 is directly connected, Serial1/0

D       10.1.12.0 [90/2681856] via 10.1.13.1, 00:00:08, Serial1/0

 

 

And, we get the change as expected in R1 routing table:

 

     172.16.0.0/24 is subnetted, 4 subnets

D       172.16.4.0 [90/2297856] via 10.1.13.3, 00:00:12, Serial1/1

D       172.16.1.0 [90/2297856] via 10.1.13.3, 00:00:12, Serial1/1

D       172.16.2.0 [90/2297856] via 10.1.13.3, 00:00:12, Serial1/1

D       172.16.3.0 [90/2297856] via 10.1.13.3, 00:00:12, Serial1/1

     10.0.0.0/24 is subnetted, 2 subnets

C       10.1.13.0 is directly connected, Serial1/1

C       10.1.12.0 is directly connected, Serial1/0

 

Now, we see all the networks in 172.16.0.0 network. The goal of no auto-summary has been achieved.

Let’s have a look now at routing table for R2:

 

172.16.0.0/24 is subnetted, 4 subnets

D       172.16.4.0 [90/2809856] via 10.1.12.1, 00:00:25, Serial1/0

D       172.16.1.0 [90/2809856] via 10.1.12.1, 00:00:25, Serial1/0

D       172.16.2.0 [90/2809856] via 10.1.12.1, 00:00:25, Serial1/0

D       172.16.3.0 [90/2809856] via 10.1.12.1, 00:00:25, Serial1/0

     10.0.0.0/24 is subnetted, 2 subnets

D       10.1.13.0 [90/2681856] via 10.1.12.1, 00:05:44, Serial1/0

C       10.1.12.0 is directly connected, Serial1/0

 

What?? Wait a minute, we only disabled auto summary at R3 not at R1 then why we are getting all the networks advertised by R3? Why R1 is not advertising 172.16.0.0 network as one big 172.16.0.0/16 network?

 

Here is the catch guys, an EIGRP router will only summarize those routes which are being advertised by itself using network command. It cannot summarize those networks which it is learning from other routers.

 

This is one concept which is very easy to overlook when you have not experienced this yourself. This is something you only get to know when you are doing it yourself.

 

Related Information

Comments
Hall of Fame Cisco Employee

Hello,

Yes, you are correct.

The information about EIGRP auto-summarization is somewhat scarce but it does exist. Regarding issues with auto-summarization of external routes, this article talks about it:

http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094cb7.shtml#summaryofexternals

In short, routes will be autosummarized only if there is a directly attached network that belongs to the same major network as the routes themselves, and is added to the EIGRP process using the network statement. This applies both to internal and external routes. So, slightly contrary to what you are stating, EIGRP can summarize networks that have been learnt via EIGRP but some additional checks are made before the auto-summarization takes place.

In your example, if you added a loopback on R1 with the IP address of 172.16.5.1/24 and added it to the EIGRP process, the R1 would then start autosummarizing the entire 172.16.0.0/16 range towards R2. After this change, R2 should see only 172.16.0.0/16 in place of 172.16.[0-5].0/24. If you test this scenario, you may want to use the clear ip eigrp neighbor soft command on either R1 or R2 because I have seen a bug in EIGRP related to not removing the component routes after advertising a summary.

The fact is, however, that the auto-summarization is so crude in these days that it is almost always turned off, and instead, manual summarization is configured.

Best regards,

Peter

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