EIGRP Auto-Summary

This topic is to discuss the following lesson:

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!

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.

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!

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.


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

Very useful indeed! Thank you Rene, I learn something new from each one of your articles

this a great article and explaination


Hi Rene

Thank you that was a useful lesson.
There is an important thing that you didn’t explicitly mention and is important.

  • The eigrp router will only announce an auto-summary out an interface if the classful network of the interface is different from the classful network of the interface it learnt the network from. In other words if it has 2 interfaces and both are in the same network class, it will never send an auto summary out (this is how I understood the cisco info, haven’t labbed it).
    Cisco says it this way "When a router has multiple working interfaces, and those interfaces use IP addresses in different classful networks, the router advertises a summary route for each classful network on interfaces attached to a different classful network.

What I took away from the lesson is:
-A router will only send out auto-summaries of connected networks (never learnt)
-A router will only auto-summarize between network classes.


Hi Rene
I got this “EIGRP auto-summary will only create summary routes for directly connected networks, not for routes you learn from other EIGRP routers.”
But what about OSPF auto-summary route. Does it do the same thing ?
Thank you so much .

Hi Heng,

OSPF doesn’t support auto-summary like EIGRP does. You can only use summarization on ABR and ASBR routers, and you’ll have to configure the summary route yourself.

Hi guys,

so can we say that the auto-summary command is the same for EIGRP and RIP?
When enabled, routes will be summarized to the class A, B or C network and no mask will be send with the update message?


Hello florian

Essentially, yes, that is correct. However, EIGRP unlike RIP will automatically install a discard route in the local routing table. The discard route is one that has as an exit interface the Null0 interface.

I hope this has been helpful!


Hi Laz,

thanks for your answer!