EIGRP Operation and Neighbors

Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco proprietary routing protocol that uses a hybrid of distance vector and link-state routing protocols.

EIGRP routers discover neighbors through the use of multicast and unicast hello packets. EIGRP neighbors must have matching configuration parameters such as the same autonomous system number (ASN) and authentication key.

EIGRP routers exchange topology information with their neighbors through EIGRP messages, including hello, update, query, and reply messages. The updates include only the changes to the routing table, which reduces network traffic and speeds up convergence.

EIGRP supports multiple network layer protocols, including IP, IPv6, and AppleTalk. EIGRP automatically summarizes routes at network boundaries, but this can be disabled with the “no auto-summary” command.

EIGRP uses a metric called “feasible distance” to determine the best path to a destination. The feasible distance is calculated based on the minimum bandwidth, total delay, reliability, load, and maximum transmission unit (MTU) of the path. The successor route is the path with the lowest feasible distance. EIGRP allows for unequal-cost load balancing, where traffic can be distributed over multiple paths with different metrics. EIGRP also supports route summarization, authentication, and route filtering through access lists.