3.17 EIGRP Queries

Note the following troubleshooting commands useful in investigating EIGRP query activity. When queries are being sent by a router, its show ip eigrp topology output will show the destination as Active (A).

Here are the conditions for sending EIGRP queries:

1. Queries are sent when a route is lost and there is no Feasible Successor in the EIGRP topology table.
2. Queries are sent to all EIGRP neighbors, except the interface of the Successor.
3. The neighbor will either (a) reply with routing information about the destination, or if none is available will (b) send a query to its neighbors, excepting the interface which received the update from the original router (split horizon).

Here are the following actions a router will take when DUAL behavior changes in relation to a specific route/prefix:

• The router attempts to resolve the issue locally; it will check its topology table for FS (Feasible Successors) and if one is found, it will immediately be inserted into the routing table and life is good.
• If no FS exist, the prefix will be marked as Active (A, versus P or Passive, the normal stable state of EIGRP) and begin the neighbor query process.
• The prefix stays A until one of the following occurs:
  • All queries are responded to
  • The active timer expires
• If no new FS is found through the query process, mark the destination as unreachable.

Here is an example of the debug eigrp fsm command in action when a user shuts the Loopback 0 interface on router BR1:

You can imagine how the process can be unwieldy if a route tree is exceptionally large. The means to mitigate these issues can be found in route summarization or use of the EIGRP stub routing feature.

(Also, keep in mind that although queries are generally multicast, there are special circumstances where they will be sent as a unicast.)