There are a few Codes in the EIGRP Topology Table. This table can be shown with the command #show ip eigrp topology. There are 3 important codes to understand which are Active (A), Passive (P) and replay Status (r).
P : This code shows that the route is a Passive route which means that the route is NOT being calculated by the DUAL algorithm (not in process of calculating). Therefore, a route in this state is usable.
A : This code shows that the route is in an Active state which means that the route is currently being calculated by the DUAL algorithm. While being in this process, the route is not available for use. The DUAL calculation is usually short, so this Active state is also short and soon changes to the Passive Route (P). However, there are certain conditions when the route stays in this condition which is the SIA (Stuck In Active) condition. SIA happens when the router loses its successor and does not have a feasible successor. The router will send queries to the neighboring routers for the feasible successor and if there is no answer for a very long time from them, the neighbor relationship will be reset and SIA state will occur.
r : This code means that the query has been sent, but there is no response from the neighbor.
There are 4 possibilities why a route goes to SIA:
1. The link is a unidirectional link. The main router can send a packet, but it is not able to receive a packet back.
2. The destination router resource is not available. This can happen when the router's CPU processing processor is very high.
3. The destination router memory is corrupted, so that it cannot reply the query.
4. The link is a low quality link so that the reply cannot be received by the original router.
-- 1 February 2011 --
P : This code shows that the route is a Passive route which means that the route is NOT being calculated by the DUAL algorithm (not in process of calculating). Therefore, a route in this state is usable.
A : This code shows that the route is in an Active state which means that the route is currently being calculated by the DUAL algorithm. While being in this process, the route is not available for use. The DUAL calculation is usually short, so this Active state is also short and soon changes to the Passive Route (P). However, there are certain conditions when the route stays in this condition which is the SIA (Stuck In Active) condition. SIA happens when the router loses its successor and does not have a feasible successor. The router will send queries to the neighboring routers for the feasible successor and if there is no answer for a very long time from them, the neighbor relationship will be reset and SIA state will occur.
r : This code means that the query has been sent, but there is no response from the neighbor.
There are 4 possibilities why a route goes to SIA:
1. The link is a unidirectional link. The main router can send a packet, but it is not able to receive a packet back.
2. The destination router resource is not available. This can happen when the router's CPU processing processor is very high.
3. The destination router memory is corrupted, so that it cannot reply the query.
4. The link is a low quality link so that the reply cannot be received by the original router.
-- 1 February 2011 --
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