One of the final things I showed in the last article was the behaviour of a router trying to find a lost route when there was no successor. The router “went active” on the route to find a new successor. Cisco has a document on the Stuck in Active problem that explains the whole process, including a problem (and likely test question) about what “Stuck In Active” means. Basically, if a router tries to find a successor, but doesn’t get an answer back within 3 minutes, it flags this error.
To quote the document:
Routes that have a valid successor are said to be in a “passive” state. If, for some reason, a router loses a route through its successor and does not have a feasible successor for that route, then the route transitions to an “active” state. In the active state, a router sends queries out to its neighbors requesting a path to the lost route.
When an EIGRP neighbor receives a query for a route, it behaves as follows:
* If the EIGRP topology table does not currently contain an entry for the route, then the router immediately replies to the query with an unreachable message, stating that there is no path for this route through this neighbor.
* If the EIGRP topology table lists the querying router as the successor for this route and a feasible successor exists, then the feasible successor is installed and the router immediately replies to the query.
* If the EIGRP topology table lists the querying router as the successor for this route and a feasible successor does not exist, then the router queries all of its EIGRP neighbors except those sent out the same interface as its former successor. The router will not reply to the querying router until it has received a reply to all queries that it originated for this route.
* If the query was received from a neighbor that is not the successor for this destination, then the router replies with its successor information.
To boil it down, EIGRP usually keeps a feasible successor route for prefixes in the routing table where the advertised distance from the alternate path is lower than the feasible distance from the preferred path (successor). If not, DUAL has to be run to find the route when the successor goes away.
Upon losing the route and finding no feasible successor, the router broadcasts to its neighbours asking for another way. If the router receiving the broadcast doesn’t have one, it replies with a negative acknowledgement. If the router receiving the broadcast has the querier as the successor, then it knows the path has gone down and goes through the process itself. (ie “Why are you asking me? I thought you knew!”) Note that if the router has a feasible successor, it can immediately use that, and pass the information along.
In a large network, this may take time. If it takes over three minutes, there is probably a problem, which is where the stuck in active condition happens.
EIGRP FAQ
ref: -
http://ccnprecertification.com/2004/08/15/eigrp-stuck-in-active/
