EIGRP Auto-Summary

EIGRP supports automatic summarization which as the name implies means that it will automatically advertise summary routes to other EIGRP routers.

How does it work? EIGRP is not going to automatically create the most optimal summary routes for you…

EIGRP will check if you have any subnets that fall within the range of your network command(s) and if so, it will advertise the classful A, B or C network to its neighbors. It will only summarize routes from subnets on its own interfaces, not the ones you learn from other routers.

Instead of advertising specific subnets we will only advertise class A,B or C networks. I’ll walk you through this, here’s the first topology we will use to demonstrate all of this:

EIGRP Auto Summary Example

Above we have two routers, R1 has two loopback interfaces with subnets that we’ll advertise in EIGRP.

Let’s configure EIGRP:

R1(config)#router eigrp 1
R2(config)#router eigrp 1

In the configuration above I enabled auto-summary on R1 which is advertising its loopback interfaces.

Depending on your IOS version, auto-summary might be enabled or disabled by default.

Let’s take a look at the routing table of R2:

R2#show ip route eigrp 
D [90/409600] via, 00:03:40, FastEthernet0/0
D [90/409600] via, 00:00:26, FastEthernet0/0

Since auto-summary is enabled, R1 is advertising /8 (class A network) and /16 (class B network).

In our small network this will work fine. When R2 wants to reach /24 or /24 then it will send its packets towards R1. We will run into issues however when we add a third router that will advertise the same summary route. Here’s an example:

EIGRP Discontigious Network

Above I have added R3 behind R2 and it has subnet /24 on a loopback interface. When we enable auto-summary on R3 then it will advertise subnet /24 as the classful network /8. This will be a problem since R2 will think it has two equal paths to reach /8. We call this a discontigious network. Let me demonstrate this:

R2(config)#router eigrp 1
R3(config)#router eigrp 1    

Above I enabled EIGRP on the link between R2 and R3 and we use the network command to advertise /24, auto-summary is enabled on R3. Let’s see what R2 thinks of this:

R2#show ip route eigrp 
D [90/409600] via, 00:00:03, FastEthernet0/1
               [90/409600] via, 00:00:03, FastEthernet0/0
D [90/409600] via, 00:00:13, FastEthernet0/0

R2 believes it can reach /8 through R1 or R3. This will cause issues…when R2 wants to reach /24 then it might send packets to R3 and when it tries to reach /24 then they might end up at R1.

How can we solve this? There are two solutions:

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Forum Replies

  1. awesome lessons. especially the

    “EIGRP auto-summary will only create summary routes for directly connected networks, not for routes you learn from other EIGRP routers.”

    i just learned it here.

    thanks rene!

  2. Good to hear you like it! I think that’s one of the things that is important to know, most people assume EIGRP will just summarize anything when you use auto-summary.

  3. Are you testing our troubleshooting skills as we move through the lessons? You started us off configuring routes in AS 12 for EIGRP and then moved onto AS 1. My head was spinning for a few minutes when I couldn’t replicate the console output that you were getting. When I made sure everything was in AS 12 it all came together! :slight_smile:

    Appreciate all of your hard work, doing these labs is really putting it all together for me. Keep it up, dude!

  4. Hi Alex,

    Not intentionally but sometimes this happens :slight_smile: It’s a good way to learn that the AS number has to match hehe.

    Anyway I just fixed it, we use AS 1 number everywhere now.


  5. i have read so many references, but you r born to teach!!! easly,understanding, and practically. thank you

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