EIGRP Auto-Summary

Lessons Discussion
1

This topic is to discuss the following lesson:

2

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!

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3

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.

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4

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!

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5

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.

Rene

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

7

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

8

Rene
this a great article and explaination

thanks

9

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.

Thanks
Raul

10

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 .

11

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.

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12

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?

Thanks
Florian

13

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!

Laz

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Hi Laz,

thanks for your answer!

Regards
Florian

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image
image851Ă—231 3.42 KB

in the above pic there should not be any problem with the auto summary , because the major network boundary is same …Right ?.

can u please explain about the major network boundary little bit ?

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Hello Narad

Actually, there will be a problem here. WIth auto-summary enabled, R1 will advertise the 1.0.0.0/8 network to R2. Similarly, R3 will advertise the 1.0.0.0/8 network to R2. Therefore, R2 will install two equal-cost routes to 1.0.0.0/8 via R1, and R3, much like the example in the lesson.

In such a scenario, auto summary should be turned off.

So what is a “major network boundary”? Well, as far as EIGRP and auto summary is concerned, it is the same thing as a classful network boundary. In other words, if R1 advertises its networks with auto summary on, it will use the classful subnet mask of /8. Thus, the major network boundary is 1.0.0.0/8, which ranges from 1.0.0.0 to 1.255.255.255. Anything beyond this would be considered outside of the major network boundary. Similarly, an auto summarized advertisement of the 172.16.5.3/24 network would be 172.16.0.0/16. Anything outside of this range, would be considered outside of the major network boundary.

I hope this has been helpful!

Laz

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"Actually, there will be a problem here. WIth auto-summary enabled, R1 will advertise the 1.0.0.0/8 network to R2. Similarly, R3 will advertise the 1.0.0.0/8 network to R2. Therefore, R2 will install two equal-cost routes to 1.0.0.0/8 via R1, and R3, much like the example in the lesson."

As you said , When R3 and R1 will advertise the 1.1.5.0/24 and 1.1.1.0/24 to r2 , R2 will not do load balancing as ECMp bcz the R2 interface has configured the IP address as /24 and that the same major network as R3 and R1 advertised prefix so there will be no problem with the auto summary .

is not it ??

18

Hello Narad

If you enable auto-summary, then you will have a routing table in R2 that is something like this:

R2#show ip route 
D    1.0.0.0/8 [90/409600] via 1.1.4.2, 00:00:03, FastEthernet0/1
               [90/409600] via 1.1.3.1, 00:00:03, FastEthernet0/0
D    172.16.0.0/16 [90/409600] via 192.168.12.1, 00:00:13, FastEthernet0/0
C    1.1.3.0/24 is directly connected, FastEthernet0/0
C    1.1.4.0/24 is directly connected, FastEthernet0/1

(this is not exact output, I’ve put this together myself so it may not appear exactly like this, but you get the idea…)

So you see, any traffic going to 1.1.3.0/24 or 1.1.4.0/24 will work fine, because these are directly connected networks. The summary route will be ignored because directly connected routes have an administrative distance (AD) of 1, which means they take precedence over EIGRP-learned routes with an AD of 90.

However, if you want to reach 1.1.5.0/24 or 1.1.1.0/24, you can see that it is the summary route that will be matched. Because the summary route contains two next hops with equal costs, packets will be attempted to be load-balanced across both links, so communication with these networks will fail.

I hope this has been helpful!

Laz

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Thanks Laz, This explanation of Major Network Boundary helped me understand the concept of Discontiguous networks which was not clear to me until now.

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20

bonjour Monsieur RENE , je ne comprends pas cette phrase

Regardez de près cependant… même si R2 est configuré avec un résumé automatique, il ne résumera pas le sous-réseau 1.1.1.0 /24 au réseau classful 1.0.0.0 /8. En effet, le récapitulatif automatique EIGRP créera uniquement des itinéraires récapitulatifs pour les réseaux directement connectés, et non pour les itinéraires que vous apprenez à partir d’autres routeurs EIGRP.