EIGRP Cheat Sheet

EIGRP is a Cisco propriety routing protocol.

EIGRP stores all route information in it's topology table. From there it chooses the primary best route and puts it in the routing table. This best route is called the Successor route. It's the primary route used and is installed in the routing table. Any backup routes that could be used in the AS without causing loops are called feasible successors. These remain in the topology table.

EIGRP uses the Diffusing Update Algorithm to update the routing table. When a primary link fails EIGRP will update the routing table with the next best Feasible Successor. It does this without having to talk to neighboring routers which makes for rapid convergence. The Neighbor table lists adjacent routers.

If there is no feasible successor the router will send multicast EIGRP queries to neighbor routers and calculates a new successor based on their replies.

EIGRP routes are marked as (P)assive which means good. If the primary route fails the route is marked as (A)ctive meaning it is trying to find another route. This happens when the successor has failed and there was no feasible successor.

By default EIGRP put up to 4 equal paths into the routing table.

no ip auto-summary - Turn off auto-summarization for discontigous networks, (which is mostly the case) 

debug ip eigrp - debugging

show ip eigrp topology - View Topology

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