EIGRP routing

EIGRP capabilities and operations

EIGRP stores all neighbor routing tables for quick adaptation to alternate routes. If a suitable route does not exist, EIGRP queries its neighbors to discover an alternate route. It propagates these queries through the network to locate a route. EIGRP's support for variable-length subnet masks allows routes to be automatically summarized on a network boundary. Additionally, EIGRP can be configured to summarize any bit boundary at any interface.

EIGRP sends partial updates when route metrics change, limiting propagation to necessary routers, which minimizes bandwidth use compared to protocols that send periodic full updates.

To learn about routers on attached networks, EIGRP uses neighbor discovery through multicast hello packets to announce their presence on the network. When a new neighbor is detected through hello packets, a topology table exchange occurs.

EIGRP hello packets are normally sent as multicast messages and do not require explicit replies. For manually configured static neighbors, hello packets are sent as unicast messages to the specified neighbor.

Once this neighbor relationship is established, routing updates are exchanged only on topology changes. The relationship persists through regular multicast hello packets, with devices expected to respond within advertised hold times. Hold time is the time within which threat defense can expect to receive a hello packet from that neighbor. If the device does not receive a hello packet within the advertised hold time, it considers the neighbor unavailable.

EIGRP employs mechanisms like neighbor discovery/recovery, Reliable Transport Protocol (RTP), and Diffusing Update Algorithm (DUAL) for route computations. DUAL retains all routes to a destination, selecting the least-cost route for packet forwarding while retaining others in case of network changes. If the main route fails, another route is chosen from the feasible successors. A successor is a neighboring router that is used for packet forwarding that has a least-cost path to a destination. DUAL utilizes a feasibility condition to ensure that the path is not part of a routing loop.

If a feasible successor is not found in the topology table, a route recomputation takes place. During route recomputation, DUAL queries the EIGRP neighbors for a route. The query is propagated to successive neighbors. If a feasible successor is not found, an unreachable message is returned.

During route recomputation, DUAL marks the route as active. By default, threat defense waits for three minutes to receive a response from its neighbors. If the device does not receive a response from a neighbor, the route is marked as stuck-in-active. All routes in the topology table that point to the unresponsive neighbor as a feasible successor are removed.