Route summarization

[per.nielsen]

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

[pkhatri · Accepted solution]

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

[per.nielsen]

That was a very good explanation

Thanks a lot

[Harold Ritter]

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,