EIGRP auto summary question

Answered Question
May 5th, 2012

                   Hi all,

I have question about EIGRP auto summary.

Lets say R1 is connected to R2 and R2 is connected to R3.

R2 and R3 have auto summ on by default.

R1 has no auto summary configured.

If R1 advertise about its Lan network 10.10.10.0/24

Router EIGRP 100

network 10.0.0.0

no auto summary

When router 10.10.10.0 reaches R3 will it be shown as or 10.0.0.0/8 classfull address  in R3s routing table?

Thanks

MAhesh

I have this problem too.
0 votes
Correct Answer by Mohamed Sobair about 1 year 11 months ago

Mahesh,

Here is the straight forward concept for Eigrp Auto-Summary that you wouldnt forget in the future:

Auto-summary:  When a router running Eigrp have Auto-summary enabled, this router summarizes all network updates between (DIFFERENT  MAJOR  NETWORK  BOUNDARIES).

This means, if the major network between different routers running eigrp with auto-summary enabled are the SAME, then it WILL NOT summarize any network updates.

Lets take a look at your example:

R1 has no auto-summary and advertises Network 10.0.0.0/24 to R2, if the Link between R2 and R3 is from the same major Network 10.x , for example 10.1.1.1/24, then R2 WILL NOT advertise a summary route of 10.0.0.0/8 to R3, Even though R2 has Auto-Summary enabled. (because they are not in DIFFERENT Major Network Boundaries).

However, if the Major Network between R2 and R3 is different than 10.x, then it will advertise a summary route of 10.0.0.0/8 to R3. for instance, if the Link between R2 & R3 is  from the Network 192.168.x or 172.16 or Whatever but NOT the 10.x, then a Summary of 10.0.0.0/8 would be sent from R2 to R3 , and R3 would have 10.0.0.0/8 in its routing table.

I hope this concept is clear for you and let us know if you have further inquiries,

Regards,

Mohamed

Correct Answer by maxime.gerges about 1 year 11 months ago

Hello !

To answer your question directly I might say that R3 would have a route for 10.10.10.0 with a /24 prefix-length and not /8 even if R3 and R2 are configured with auto-summary as default.

A little explanation about the auto-summary feature : if a router has at least 2 interfaces in different classfull network, the default behavior is to advertise a classfull summary for these two networks (if matched by a network command...).

Auto-summary feature has no effect on route being learned by a router but only when the router sends an update.

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Correct Answer
maxime.gerges Sat, 05/05/2012 - 12:28

Hello !

To answer your question directly I might say that R3 would have a route for 10.10.10.0 with a /24 prefix-length and not /8 even if R3 and R2 are configured with auto-summary as default.

A little explanation about the auto-summary feature : if a router has at least 2 interfaces in different classfull network, the default behavior is to advertise a classfull summary for these two networks (if matched by a network command...).

Auto-summary feature has no effect on route being learned by a router but only when the router sends an update.

mahesh18 Sat, 05/05/2012 - 13:44

Hi Maxime,

Thanks for reply.

so if R1 has 2 interfaces and Lan interface has Class A IP.

Wan interface has Class B   IP.

Now as they are in different classfull network and as R1 has been configured with no auto summary so R1 will advertise the

classless network which is /24 for in our case here right?

Thanks

Mahesh

Correct Answer
Mohamed Sobair Sat, 05/05/2012 - 13:18

Mahesh,

Here is the straight forward concept for Eigrp Auto-Summary that you wouldnt forget in the future:

Auto-summary:  When a router running Eigrp have Auto-summary enabled, this router summarizes all network updates between (DIFFERENT  MAJOR  NETWORK  BOUNDARIES).

This means, if the major network between different routers running eigrp with auto-summary enabled are the SAME, then it WILL NOT summarize any network updates.

Lets take a look at your example:

R1 has no auto-summary and advertises Network 10.0.0.0/24 to R2, if the Link between R2 and R3 is from the same major Network 10.x , for example 10.1.1.1/24, then R2 WILL NOT advertise a summary route of 10.0.0.0/8 to R3, Even though R2 has Auto-Summary enabled. (because they are not in DIFFERENT Major Network Boundaries).

However, if the Major Network between R2 and R3 is different than 10.x, then it will advertise a summary route of 10.0.0.0/8 to R3. for instance, if the Link between R2 & R3 is  from the Network 192.168.x or 172.16 or Whatever but NOT the 10.x, then a Summary of 10.0.0.0/8 would be sent from R2 to R3 , and R3 would have 10.0.0.0/8 in its routing table.

I hope this concept is clear for you and let us know if you have further inquiries,

Regards,

Mohamed

mahesh18 Sat, 05/05/2012 - 14:06

Hi Mohammed,

I read you post few times and understand the fact.

Many thanks

Mahesh

ZbigniewJ Thu, 02/28/2013 - 11:08

Hello, it's been a few monts since you posted your question, but i fund it while preparing to ROUTE exam and i feel need to respond.

Well I must say Mohamed i wrong. In your case R2 will NOT advertise summary route to R3.

The official certification guide says:

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.

So learned routes are not subject of auto-summarization. They can still be summarized manually ofcourse.

Firs of all - create a lab. Ok. I did it for you:

10.10.10.0 - R1 - 192.168.12.x - R2 - 193.168.23.x - R3

R1 does not summarize, R2 does.

Lets see at R3:

R3#sh ip route

D    192.168.12.0/24 [90/307200] via 193.168.23.2, 00:42:30, FastEthernet0/0

     10.0.0.0/24 is subnetted, 1 subnets

D       10.10.10.0 [90/332800] via 193.168.23.2, 00:42:30, FastEthernet0/0

C    193.168.23.0/24 is directly connected, FastEthernet0/0

See? No summary route. Let's do an experiment - let's add loopback interface on R2 with IP of 10.1.1.1 and check R3 again:

R3#sh ip route

D    192.168.12.0/24 [90/307200] via 193.168.23.2, 00:50:45, FastEthernet0/0

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

D       10.10.10.0/24 [90/332800] via 193.168.23.2, 00:50:45, FastEthernet0/0

D       10.0.0.0/8 [90/409600] via 193.168.23.2, 00:00:35, FastEthernet0/0

C    193.168.23.0/24 is directly connected, FastEthernet0/0

Now we have a 10/8 network as R2 summarised 10.1.1.0/24 to a classfull boudary, but we can still see /24 network as well. Thats the freaky way the auto-summariazation works.

You have to remember: auto-summary feature indeed works when a router advertises prefixes between major networks, but ONLY directly connected ones. It does not affect learned networks.

EfosaOsaghae-M Fri, 02/21/2014 - 08:54

@ZbigniewJ

Thank you so much for that, (no matter how late it is).You are absolutely right! Finally a definitive answer and explanation proven by demonstration. Creating a lab to observe what actually happens is exactly what I would have done, (thanks for doing that for me :). This ends any further question and clears any doubt or confusion that myself (and I'm sure many others out there), may have had as to the subject of EIGRP auto-summary functionality.  

(oh and sorry.. meant to rate your answer a full 5-stars but mistakenly clicked 4).

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