EIGRP Successor and Feasible Successor

Feasible Distance(FD) – The Feasible Distance, or FD in short, is the historical minimum of the best metric towards that particular destination (with the history starting from scratch when the route neds to enter the Active state and then returns back to Passive state). This means that the FD may be equal to or lower than the current total best metric towards the destination, including the link to the next hop (or successor in EIGRP parlance).

Reported Distance(RD) – Reported Distance, or RD, as advertised by a particular neighbor, is its current total distance to a particular destination. It is simply the neighbor’s own distance to the destination. Our total distance via a particular neighbor is that neighbor’s RD plus the metric of the link between us and that neighbor.

The successor route is the best route to reach a subnet, based on the advertised distance (AD) from the neighbor plus the distance to reach that neighbor. This is the route which is installed in the routing table.

The feasible successor route is a route which has a higher metric than the successor route to reach a subnet but meets the feasibility condition and can be used in the event that the successor route goes down. This route does NOT get installed in the routing table but is kept in the topology table.

Successor – A successor route (think successful!) is the best route to a remote network. A successor route is used by EIGRP to forward traffic to a destination and is stored in the routing table. It is backed up by a feasible successor route that is stored in the topology table-if one is available.

Feasible successor – A destination entry is moved from the topology table to the routing table when there is a feasible successor. A feasible successor is a path whose reported distance is less than the feasible distance, and it is considered a backup route.

EIGRP will keep up to six feasible successors in the topology table. Only the one with the best metric (the successor) is placed in the routing table. The show ip eigrp topology command will display all the EIGRP feasible successor routes known to a router.

The best route is determine by your metric to each network or device.

The feasibility condition states that the AD from a neighbor must be less than the metric of the successor route (the feasible distance [FD]) because routing through a feasible successor when the AD > FD may cause a routing loop.

All other routes (non-successor and non-feasible successor) are called possibilities are kept in the topology table but are only visible when u supply the all-links argument to the show ip eigrp topology command.

By default, an EIGRP router will store only the route with the best (lowest) feasible distance in the routing table (or, multiple routes with equivalent feasible distances). However, under certain conditions, EIGRP will also maintain less-than-optimal routes in its topology table. These feasible successor routes are not used in the forwarding of traffic, but can be injected into the routing table immediately in place of a failed successor route, to avoid the querying process mentioned earlier.

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https://learningnetwork.cisco.com/thread/66483
http://packetlife.net/blog/2010/aug/9/eigrp-feasible-successor-routes/

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