Determining why the route is active is not necessarily a difficult task.
Sometimes, the route that constantly is going active could be due to a flapping
link. Or, if the route is a host route (/32 route), it is possible that it is
from a dial-in connection that gets disconnected. A more important and
difficult task is determining why an active route becomes stuck. Usually, an
active route gets stuck for one of the following reasons:
- Bad or congested link
- Low router resources, such as low memory or high CPU processing on the
router
- Long query range
- Excessive redundancy
By default, the stuck in active timer is only 180 seconds. If the EIGRP
neighbor does not hear a reply for the query in 180 seconds, neighbors are
reset. This adds difficulty in troubleshooting EIGRP routes stuck in active
because when an active route is stuck, there is only 180 seconds to track down
the active route query path and find the cause.
Debugs and
Verification
The tool that helps troubleshoot the EIGRP stuck in active
error is the show ip eigrp topology active command. Figure
shows sample
output from this command. This command shows what routes are currently active,
how long the routes have been active, and which neighbors have and have not
replied to the query. From the output, it can be determined which neighbors
have not replied to the query. Track the query path and find the status of the
query by hopping to the neighbors that have not replied.
As the output
indicates, the route for 20.2.1.0 is in active state and has been active for 1
minute and 43 seconds.
Query-origin is
Successor Origin, which means that the successor to this route sends the query
to this router. At this point, it has received replies from 10.1.3.1 and
10.2.4.1. The reply is infinity, which means that these two routers also do not
know about 20.2.1.0. The most important output of the show ip eigrp
topology active command is the Remaining replies section. This
router shows that the neighbor 10.1.5.2 from interface Serial 1/2 has not
replied to the query.

To proceed
further with troubleshooting, the next step is to Telnet to the 10.1.5.2 router
to see the status of its EIGRP active routes, using the same show ip
eigrp topology active command. Sometimes, the router does not list
the neighbors that have not replied to the queries under the Remaining replies
section.
Figure
shows
another example of the show ip eigrp topology active
command output. The only difference between the two outputs is the list of
neighbors that have not replied to the router.
However,
this does not mean that all of the neighbors have replied to the queries. The
neighbor 1.1.1.2 has a lowercase r next to the address of 1.1.1.2. This also
means that the neighbor has not replied to the queries.
The router has
two ways of representing neighbors that have not replied to the queries. One
way, is to have them listed under the Remaining replies: section. The other is
to have an r next to the neighbor interface IP address. When using the
show ip eigrp topology active command, the router can use
any combination of these methods to represent neighbors that have not yet
replied to the queries.

The
neighbors that have not replied to the queries are 1.1.1.2 and 10.1.5.2.
Only one of the
non-replying neighbors 10.1.5.2 is listed under the Remaining replies: section.
The other neighbor, 1.1.1.2 that has not replied is listed with the other
replying neighbor. To summarize, when issuing the show ip eigrp
topology active command, the most important part to look for is the
neighbors that have not replied to the query. To look for such a neighbor, look
for neighbors that have the r next to their interface IP address.