What does EIGRP Stuck inactive state mean?

[ranjipremandrewp]

What does EIGRP Stuck inactive state mean?

[Peter Paluch · Accepted solution]

Hi,

The EIGRP Stuck In Active, or SIA in short, means that the router is involved in a diffusing computation for a new path to some network, and this computation seems to be stalled because some expected replies have not arrived in a reasonable time.

In EIGRP, the diffusing computation is a process of asking the neighboring routers to assist in locating a replacement path to a network for which the usable path has been lost. In a diffusing computation, a router sends Query packets to its neighbors, informing them about its current distance from the destination, and asking them for their own distance. If the neighbor is not affected by the Query, it just sends a Reply packet right away with its own current distance to the destination. If the Query has affected the neighbor in such a way that it also lost the path to the destination (which could have happened if that router was using us as its next hop), it will send a Query on its own to its own neighbors. After sending a Query to its neighbors, a router must wait for Reply packets from all routers to arrive, and only then it can make the selection and start sending Replies itself. If, in this chain of events, any router has trouble getting all Reply packets its needs, the diffusing computation can not be completed, and the new path to the destination can not be established. This situation is called Stuck in Active, and is, to be honest, EIGRP's Achilles heel.

In well behaved networks, SIA states should not happen. They occur with, say, overloaded links, overutilized CPUs on routers, bugs in IOS, etc., but normally, they should not appear. By default, a SIA state is declared if the diffusing computation could not be completed in 3 minutes (180 seconds).

Feel welcome to ask further!

Best regards,

Peter

[gd0056277]

Hello Peter,

I know its a fairly old post but I just feel that asking a question here would be more relevant than another post. I just want to understand here that how crucial is the part where you say " If the neighbor is not affected by the Query, it just sends a Reply packet right away with its own current distance to the destination."

So lets say we have a scenario where a link goes down along a path which is neither a Successor or Feasible Successor. So in essence, no one is effected from the point of view of a particular destination. So how would the routers along this path respond ??

Cheers

G

[Peter Paluch]

Hello,

So lets say we have a scenario where a link goes down along a path which is neither a Successor or Feasible Successor. So in essence, no one is effected from the point of view of a particular destination. So how would the routers along this path respond ??

Consider the following exhibit.

Let's focus on R2, and assume that the R2/R3 link goes down. From R2's viewpoint:

• The path from R2 to Network A is not affected by this event, so it will not start querying for it at all. This is the case you're basically asking about. The affected path did not have an impact on the distance of R2 toward Network A, so that's why it does not care.
• The path from R2 to Network B is obviously affected, so R2 will send out a Query asking for updated distances of its remaining neighbors toward Network B. Ultimately, this Query will be received by its only remaining neighbor, R1, and because R1 has no further neighbor to ask itself while the Query indicates an unreachable distance, it will itself send a Reply with infinite distance, after which both R1 and R2 remove the Network B from their topology and routing tables.

In your question, you went slightly wrong in asking about a situation that by definition can not occur: If a failure on a path does not affect a particular network, routers won't be querying for that network in the first place.

Does this make any sense?

Best regards,
Peter

[gd0056277]

Makes a lot of sense...got caught up in too any words..lol.. Thanks a lot for the clarification.

Cheers

G