Friday, November 5, 2010


EIGRP is simple to set up, but that can also be a bad thing. It's not uncommon for people to have EIGRP running without good design, which can end up destroying the network.

We can eavesdrop on EIGRP neighbors using the debug command:
debug eigrp packets query
debug eigrp packets reply

To see how the query messages work, we can make an interface go down. To do this, we can shut down the lo0 interface:
in lo0

Immediately after the Line Protocol goes down, query messages are sent to all neighbors to look for alternate paths to the lo0 network. However, since no one else has it, you will quickly see negative replies from other routers.

Query messages will go to every neighbor on every interface. Whoever receives a Query will send it to other neighbors if it doesn't have the route. This results in something that looks like a broadcast storm if there are looped interfaces.

All replies must be received before the router can make a decision, even if one of the replies has a backup route. If the network is very big, it would be a long time before the queries would come back. This results in a symptom known as SIA (Stuck-in-Active). After 3 minutes, all neighbor relationships are reset, causing a downtime before operation is possible.

There are two ways to solve SIA:
1) Summary Routes
2) Stub Configuration

If you have a set of routes from 172.16.[0-7].0/24 and you've configured a summary of and one of the 172.16.x.0/24 networks go down, the query would still go out, but routers would assume that the original path is the only way to the network and would give a negative reply instead of forwarding the query along.

You can also configure a network as a stub. A stub is simply a network with 1 exit only, so if a network is configured as a stub, it will assume that the link it has is the only one exit and would not query anything else.

To configure a stub, we'll use:
router eigrp 1
eigrp stub

If a router is configured as stub, no queries will be sent to it if a route goes down. A stub router simply means that the router will only have the routes it tells you about and nothing else.

To configure the router to not even advertise anything, similar to a passive-interface on RIP, use:
eigrp stub receive-only

By default, K1 and K3 is set to 1 for Bandwidth and Delay. If routers have different K Values set, neighbors will not come up properly. In 12.3(2)T, EIGRP has a new feature known as Graceful Shutdown. Whenever the neighbor wants to go down, it would send a Graceful Shutdown message to let the other routers remove it from the tables immediately. Graceful Shutdown (Goodbye Message) is simply a Hello message with all K Values set to 255. Older routers would log it as "K Value Mismatch" while the newer routers would see "Goodbye Message".

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