EGRP querys and edge routers

Answered Question
Jul 23rd, 2008

Hi

I'm quoting from Cisco's Building Cisco Scalable Internetworks book, chapter on EIGRP:

"If a query reaches the edge of the autonomous system, the original query is answered. However, then the edge router initiates a new query in the other autonomous system." pp120.

I couldn't find this info so I'm confused about what would this answer from the edge router actually contain?

Thanks

-Assaf

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Correct Answer by Giuseppe Larosa about 8 years 4 months ago

Hello Assaf,

this is a scalability problem of EIGRP when dealing with multiple EIGRP AS domains.

The query reachs the ASBR router that answers the query back.

The real trouble is that the same router will start a query for the same prefix in the other EIGRP domain simply moving the problem on the other side.

The best way to control this is to summarize at the edge ASBR router so that it will not propagate the query to other AS or so that the first routers will reply to the sender.

What really contains the answer to the original query I don't know, probably the edge router in an attempt to speed convergence sends a reply back in the hope to receive an answer to its own query.

So the edge router says I can reach the subnet but it is false.

However, this behaviuor is clearly a wrong one: EIGRP has its own topology database where it makes distinction about internal routes and external routes so the propagation to another EIGRP AS system could be avoided with a change in EIGRP implementation.

Before discovering this problem using multiple EIGRP domains was thought to provide more scalability and was a design guideline.

Now, it isn't recommended or at least advice to use summarization at the AS border is given.

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Thu, 07/24/2008 - 00:12

Hello Assaf,

this is a scalability problem of EIGRP when dealing with multiple EIGRP AS domains.

The query reachs the ASBR router that answers the query back.

The real trouble is that the same router will start a query for the same prefix in the other EIGRP domain simply moving the problem on the other side.

The best way to control this is to summarize at the edge ASBR router so that it will not propagate the query to other AS or so that the first routers will reply to the sender.

What really contains the answer to the original query I don't know, probably the edge router in an attempt to speed convergence sends a reply back in the hope to receive an answer to its own query.

So the edge router says I can reach the subnet but it is false.

However, this behaviuor is clearly a wrong one: EIGRP has its own topology database where it makes distinction about internal routes and external routes so the propagation to another EIGRP AS system could be avoided with a change in EIGRP implementation.

Before discovering this problem using multiple EIGRP domains was thought to provide more scalability and was a design guideline.

Now, it isn't recommended or at least advice to use summarization at the AS border is given.

Hope to help

Giuseppe

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