EIGRP query messages, the good the bad the ugly.

If a network goes down, EIGRP will send out query messages to its neighbors to find an alternate route.  EIGRP will do this for 180 seconds (three minutes) it will keep sending the query messages even if a path is reported within that time frame, the path will simply set in queue until all queries have been answered.   While this message waits the link can become “stuck in active” and after the 3 minute period all neighbor relations will be torn down and the link will become active, afterward the rest of the neighbor relations are restored.

This causes an over burden on the router processors and could **ADD250X250**  also saturate low bandwidth links not to mention a complete network “bounce” of all neighbor relations.

There are two ways to alleviate this problem… the first being the best.

1)   Summarization of routes

When routes are summarized and sent out, it tells the other neighbors that I know about this network, if you need to get this network come to me.   If a link is lost on a query message will be sent to its neighbor but the reply will be no, you told me that you knew of all these routes.  The query will stop and the router will not ask any of its neighbors.

2) Make routers R3 through R6 stub routers

r1#>config t

router eigrp 1

eigrp stub

If you hit the question mark before hitting return you will notice a variety of sub commands to choose from.   By default the command will be connected and summary.

Keep in mind that passive interface does not work the same as it does in RIP, in EIGRP it exempts the interface and will not form neighbors at all.

When we lose a link to the no quires will be sent to the neighbors, simply because the router already knows that they are all stubs and told us that nothing else exists beyond them.

To fully understand this try setting this up in GNS3 and run some EIGRP debug commands.  Keep in mind that by default the EIGRP debug will capture every freaking message sent so to you need to tighten it up a bit by only focusing on the specific message you want to capture. “debug eigrp packet query reply”

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