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.
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