About EIGRP Routing

Enhanced Interior Gateway Routing Protocol (EIGRP), developed by Cisco, is an enhanced version of IGRP. Unlike IGRP and RIP, EIGRP does not send out periodic route updates. EIGRP updates are sent out only when the network topology changes. Key capabilities that distinguish EIGRP from other routing protocols include fast convergence, support for variable-length subnet mask, support for partial updates, and support for multiple network layer protocols.

A router running EIGRP stores all the neighbor routing tables so that it can quickly adapt to alternate routes. If no appropriate route exists, EIGRP queries its neighbors to discover an alternate route. These queries are propagated until an alternate route is found. EIGRP support for the 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 does not make periodic updates. Instead, it sends partial updates when the metric for a route changes. Propagation of partial updates is automatically bounded such that only those routers that need the information are updated. As a result of these two capabilities, EIGRP consumes significantly less bandwidth than IGRP.

To dynamically learn of other routers on directly attached networks, threat defense uses neighbor discovery. EIGRP routers send out multicast hello packets to announce their presence on the network. When the EIGRP device receives a hello packet from a new neighbor, it sends its topology table to the neighbor with an initialization bit set. When the neighbor receives the topology update with the initialization bit set, the neighbor sends its topology table back to the device.

The hello packets are sent out as multicast messages. No response is expected for a hello message. Statically defined neighbors is an exception to this rule. If you manually configure a neighbor, hello messages, routing updates, and acknowledgments are sent as unicast messages.

Once this neighbor relationship is established, routing updates are not exchanged unless there is a change in the network topology. The neighbor relationship is maintained through the hello packets. Each hello packet received from a neighbor includes a hold time. 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 from that neighbor within the hold time advertised by that neighbor, the device considers that neighbor to be unavailable.

EIGRP uses neighbor discovery/recovery, Reliable Transport Protocol (RTP), and Diffusing Update Algorithm (DUAL) for route computations. DUAL saves all routes to a destination in the topology table, and not just the least-cost route. The least-cost route is inserted into the routing table. The other routes remain in the topology table. 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. A feasibility calculation ensures 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 feasibility successor are removed.