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Route summarization

Hi there

When you are configureing route summarization in OSPF or EIGRP the summary route shows up in the routing table pointing to null 0 , this is for loop prevention, Correct ???

But how could a loop happen, if you didn't have the null 0 on the summary route

thanks

per

1 ACCEPTED SOLUTION

Accepted Solutions
Purple

Re: Route summarization

Hi Per,

A loop can occur if the device generating the summary also has a default route in the routing table and the summary-address is a super-set of the routes present in the summarising device.

Consider the following:

A -- B -- C

B has the following networks in the routing table:

10.1.1.0/24

10.1.2.0/24

10.1.3.0/24

10.1.4.0/24

0.0.0.0 via A

Now B originates a summary network of 10.1.0.0/16 to A and C. Now C will send all traffic within 10.1.0.0/16 to B. Say that C sends some traffic to B that is destined for 10.1.9.10, since it is part of 10.1.0.0/16. If B does not have a null0 route for 10.1.0.0/16, it will use the default route to forward the traffic to A. Now, the best route for A to get to 10.1.9.10 is via the summary received from B, so it will send it back to B. That is what causes the loop.

If B has a null0 route to 10.1.0.0/16, it will drop the packet destined to 10.1.9.10 as soon as it receives it since it does not have a more specific route to it.

Hope that helps - pls rate the post if it does.

Paresh

3 REPLIES
Purple

Re: Route summarization

Hi Per,

A loop can occur if the device generating the summary also has a default route in the routing table and the summary-address is a super-set of the routes present in the summarising device.

Consider the following:

A -- B -- C

B has the following networks in the routing table:

10.1.1.0/24

10.1.2.0/24

10.1.3.0/24

10.1.4.0/24

0.0.0.0 via A

Now B originates a summary network of 10.1.0.0/16 to A and C. Now C will send all traffic within 10.1.0.0/16 to B. Say that C sends some traffic to B that is destined for 10.1.9.10, since it is part of 10.1.0.0/16. If B does not have a null0 route for 10.1.0.0/16, it will use the default route to forward the traffic to A. Now, the best route for A to get to 10.1.9.10 is via the summary received from B, so it will send it back to B. That is what causes the loop.

If B has a null0 route to 10.1.0.0/16, it will drop the packet destined to 10.1.9.10 as soon as it receives it since it does not have a more specific route to it.

Hope that helps - pls rate the post if it does.

Paresh

New Member

Re: Route summarization

That was a very good explanation

Thanks a lot

Cisco Employee

Re: Route summarization

you can disable this behavior with OSPF by using the "no discard-route" command under the ospf process. It might be helpful in certain cases.

You have to be careful when doing that though.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
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