How are EIGRP tables built

Dragon0x40
Dragon0x40 used Ask the Experts™
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I just finished watching a quick learning module from Cisco and they seem to be saying the neighbor table poplulates the topology table which then populates the route table.

My understanding is that the neighbor table does not even hold routes but just lists the neighbors and the status of the adjacency?

If two EIGRP routers are not neighbors then they don't exchange any routing information directly?

The topology tables holds all of the routes from the local router plus the routes advertised thru EIGRP from the directly connected neighbors? The possibilities, the feasible succesors and the successors?

How does the topology table get built? Network statements and redistribution of the routes from the local router and update packets from the directly connected neighbors?


Then the route table is populated with the best routes from the topology table?
EIGRP-Tables.pdf
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Top Expert 2004
Commented:
You know what? The absolute best thing you could do is to buy the book "Routing TCP/IP Vol. 1" Yes, it's expensive but it is the one book that I think it truly indispensable if you want to understand routing:
http://www.amazon.com/Routing-TCP-IP-1-2nd/dp/1587052024/ref=sr_1_1?ie=UTF8&s=books&qid=1278094731&sr=1-1

It will give you an extremely detailed explanation of EIGRP as well as OSPF and other IP routing protocols.

In answer to your questions, neighbors exchange topology information, from which local calculations are made and routing tables are built. Feasible successors are kept in the local topology table. Nothing happens until routers become neighbors.

Author

Commented:
thanks mikebernhardt,

I will read that book.

The diagram was confusing me because it has an arrow from the neighbor table to the topology table and I don't think route information is stored in the neighbor table. I thought that the topology and route table work together but the neighbor table was seperate but I could definitely be wrong on that.

In EIGRP terms is there a difference between being a neighbor and being adjacent?

Seems like in some protocols that two routers can become neighbors without being adjacent?

Adjacent meaning that both neighbor routers have synchronized their topology tables.
Top Expert 2004
Commented:
I was away on vacation for a week...

The neighbor table is separate, and contains information about the neighbors only, not the specific routes they advertise. In OSPF the neighbors synchronize their link state tables and then the routers do calcs to build their individual routing tables. In EIGRP once they're neighbors, they just tell each other about their routes; there is no synchronization.

Author

Commented:
So link state protocols like OSPF and ISIS become neighbors and form adjacencies so that the topology tables (link state databases) can be synchronized so that all router in the area have the exact same information in their tables.

Other distance vector protocols such as EIGRP form neighbors but do not synchronize the topology table so each router has a different topology table. (Still routing by rumor because each EIGRP router does not have the complete map of the network)

RIP routers do not become neighbors and have no topology database.
Top Expert 2004

Commented:
Correct!

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