EIGRP Queries and Stuck in Active

I’d like to tell you a little bit more about the EIGRP query process and the stuck-in-active problem. This will be helpful to understand why we use the EIGRP stub feature.

EIGRP is designed for large enterprise networks but having one big EIGRP network (5000+ prefixes and many hops) can lead to some problems:

  • Lots of EIGRP prefixes equal a large topology table and routing table.
  • Calculating the successor router will take longer if you have many EIGRP neighbors and different paths.
  • If there are many backup paths EIGRP will have to see if there are 1 or more feasible successors, this will take longer.
  • More information means our EIGRP routers have to work harder to process everything.
  • When EIGRP loses a route and there is no feasible successor the route will go from passive to active and the router starts sending queries to its neighbors.
  • EIGRP sends queries on all interfaces except the interface of the successor.

Let me describe the EIGRP query process in detail for you:

eigrp big topology failed link

In the topology above we are running EIGRP on all of the routers. Router 1 has a link failure and as a result has lost its successor route to a particular network on the left side. There is no feasible successor so the route is going from passive to active and we will send a query to router 2 and 3.

There are 2 things that can happen at this moment:

  • Router 2 or 3 has information about this particular route and will send information about it to router 1. The query process is now over.
  • Router 2 or 3 don’t know anything about this route and will send a query themselves to their neighbors router 4, 5 and router 6 and 7. Router 2 or 3 will not send a reply to router 1 until they heard a response from all their neighbors.

eigrp big topology failed link query forward

In our topology nobody has a clue which network router 1 is looking for. They will forward their queries to their own neighbors. The red arrows indicate the query packet.

eigrp big topology failed link query reply
There are no other neighbors behind router 4, 5, 6 or 7. They will send a reply to router 2 and 3 to let them know they don’t know the answer. Router 2 and 3 will send a reply to router 1 to tell them they are sorry but this is it. That’s a lot of packets for just one route that was lost right?

eigrp big topology failed link query fail

Let’s make things even more interesting. Look at my picture above and you’ll see that the reply from router 2 never makes it back to router 1. EIGRP is a reliable protocol and for each query a router sends to its neighbors it must get a reply in response within 3 minutes. If the router does not receive a reply to ALL its outstanding queries it will put the route in SIA (Stuck in Active) state and will kill the neighbor adjacency. By dropping the neighbor adjacency you will lose all the routes you learned from this neighbor which means the router will start sending queries for all those routes as well. Not a pretty sight right?

How is it possible that a reply never makes it back?

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Forum Replies

  1. great explanation ,thank you.

  2. Hi Paul,
    You generally have it right. The short answer is after 90 seconds, an SIA-Query is sent.

    The longer answer:

    Each router manages its own SIA Query and SIA Query Response management. This means that as soon as each router has put a route in Active (regardless of whether the router in question was the actual first router to start the process) and has sent a Query to its neighbors, that router also starts a timer. At 90 seconds, for each neighbor from which it has not received a reply, it will send an SIA Query to check on the status of the original que

    ... Continue reading in our forum

  3. Hello Pradeep.

    These are very good questions. In the first question, the answer is yes, even if hello packets are being recieved from a router, the SIA state will still kill the neighbour adjacency. You can think of the Stuck in Active condition as an additional condition for which neighbour adjacencies are killed.

    As for your second question, yes you’re right, the SIA reply may also fail to reach Router 1 due to congestion. Only one SIA Query is sent at 90 seconds. However, the purpose of the SIA Query and Reply feature is not only to resolve eigrp failure

    ... Continue reading in our forum

  4. Hi Ananth Maruti,

    If there is more than one feasible successor, then surely it will be listed in the topology table of EIGRP. Please remember the forumla: to have a feasible successor then the Advertised distance of feasible successor should be less than the Feasible distance of successor.

    For the 2nd part of your question, if the network in converged than the calculation of successor & feasible successor(s) is already done. In this case, if a successor route goes down, then the feasible successor takes over. In case the topology table had another feasible succ

    ... Continue reading in our forum

  5. Thanks for such a good brief .Can we have a lab where we can test it.

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