notes: EIGRP DUAL Finite state, SIA

- The lowest calculated metric to each destination will become the feasible distance (FD) of that destination.
- The feasibility condition (FC) is a condition that is met if a neighbor's advertised distance (AD) to a destination, is lower than the router's current FD to that same destination.
- If a neighbor's AD to a destination meets the FC, that neighbor becomes a feasible successor (FS) for that destination.
- Because feasible successors are always "downstream," a router will never choose a path that will lead back through itself, thus creating a loop.
- Such a path would have a distance larger than the FD.
- Every destination for which one or more feasible successors exist, will be recorded in a topology table.
- Each route after inserted, when no diffusing is taking place will be in a passive state.
- If there are two successors with the locally calculated metric equal to the FD, both routes are entered into the route table, and equal-cost load balancing will be performed.
- If a link to a successor fails(input event), or if the cost of the link increases beyond the FD (input event), the router will first look into its topology table for a feasible successor.
- If a FS is found, through local computation, it will become the successor. This occurs in the sub-second range. An update is sent to all neighbors and the route remains in the passive state.
- If a feasible successor cannot be found in the topology table, the router will begin a diffusing computation by querying neighbors for possible routes and the route will change to the active state.
- For each neighbor to whom a query is sent, the router will set a reply status flag (r) to keep track of all outstanding queries.
- The diffusing computation is complete when the router has received a reply to every query sent to every neighbor.
- If all expected replies are not received before the Active time expires, the route is declared stuck-in-active (SIA).
- At the completion of the diffusing computation, the originating router will set FD to infinity to ensure that any neighbor replying with a finite distance to the destination will meet the FC and become a feasible successor.
- Remember that queries cause the diffusing calculation to grow larger, whereas replies cause it to diminish/grow smaller.

SIA

A route is considered stuck-in-active if no response to the query has been received for a configured amount of time (default 3 minutes). After this time, the EIGRP drops all neighbors that it has not received replies from.

command:

active timer - to minimize the effect of SIA

eigrp 100
timers active time 180