EIGRP - Queries

One of the main processes that allow EIGRP to function is the use of EIGRP queries.

  1. Query Process in EIGRP: When a router, say R1, loses a route to a network and does not have a feasible successor in its topology table, it will send a query to its neighbors asking if they have a route to the lost network.

  2. Neighbor's Response to Queries: R2 will only query its downstream neighbors if it previously knew about the requested route. When R2 receives a query from R1, it will check its topology table to see if it has information about the route. If R2 has information about the route (either a feasible successor or it was the successor route), it will continue the query process by forwarding the query to its neighbors. However, if R2 has no information about the route, it will not forward the query further and will respond to R1 with a reply indicating it does not have a route.

  3. Summary Routes and Specific Route Queries: If R2 has a summary route that includes the specific route R1 is querying about, R2's response depends on whether it has a more specific route to the network in question in its topology table. If R2 only knows the network through the summary and does not have a more specific route, it will not query its neighbors for the specific route. This behavior is crucial for preventing unnecessary queries that could lead to Stuck in Active (SIA) conditions and for traffic engineering purposes, as it helps to limit the scope of queries and reduce the overall query domain.

These principles are important aspects of EIGRP's query process and how it handles specific routes versus summary routes. This understanding is vital for effective EIGRP traffic engineering and preventing routing instability.