6.3 EIGRP Data Structures
6.3.2 Route tagging with EIGRP
EIGRP classifies routes as either internal or external. Internal routes are those that originated within an EIGRP network. External routes were either learned from a different routing protocol---those that lie outside of EIGRP's Autonomous System and were learned by a router on the border between the two autonomous systems---or are static routes that have been injected into EIGRP through redistribution. All external routes are identified in the topology table and include the following information:
  • The identification number (router ID) of the EIGRP router that redistributed that route into the EIGRP network
  • The number of the Autonomous System where that route's destination resides
  • The protocol used in that external network
  • The cost or metric received from that external protocol
  • A tag that can be administratively set and used in route filtering

Route tagging gives the network administrator flexibility in establishing routing policies. This flexibility is most useful when the EIGRP network is internetworked with a policy-based routing protocol such as BGP. EIGRP routers will reject external routes tagged with a router ID identical to their own; this prevents routing loops from occurring with external routes.

The topology table also stores the identities of neighbors that are feasible successors. The concept of feasible succession was held over from IGRP. It is important to understand, however, that EIGRP's feasible succession is very different from IGRP's feasible succession. The concepts are similar, but their implementation and mechanics are very different.

In EIGRP, all neighboring routers that have an advertised composite metric (reported distance) that is less than a router's best current metric (feasible distance) for any given route are considered feasible successors to the current successor (path currently being used). If there are multiple routes with a cost equal to the best cost, they are all considered successors and are all installed in the routing table. A destination must have at least one successor before it can be moved from the topology table to the routing table.

An EIGRP router views its feasible successors as neighbors that are downstream, or closer, to the destination than it is. Whenever a change occurs in the network, which affects either its topology or even the composite metric of a single route, the set of successors and feasible successors to the affected route(s) may have to be reevaluated.

If a router loses its route through its successor, and there are no feasible successors, the route automatically goes into the active state and triggers recomputation. The router queries its neighbors, requesting new information about possible alternative paths to the impacted route. The neighboring routers must reply. Their reply can either contain information about their successors or notification that they can no longer reach the route either. The route can only return to the passive state after the router has received a reply from each of its neighboring routers and can select a successor or determine that the destination is no longer reachable.