Hello and Hold Timers
An EIGRP hello packet is not just for forming new adjacencies, but is used for checking that the neighbour is working and responsive. By default EIGRP hello packet is sent out every 5 seconds, or 60 seconds on a slow speed interface (T1 or lower)
EIGRP uses a timer called a hold timer, which is how long EIGRP determines the router to be working and reachable. The hold timer lasts as long as 3 times the hello interval, meaning the default is 15 seconds or 180 seconds on slow speed interfaces.
If a hello packet is received, the hold timer resets. If the hold timer counts to 3 times the hello interval, the neighbouring EIGRP peer is deemed to be offline or unreachable.
When the line fails, the interface will move to a down state. The move of the interface to the down state will also move any EIGRP neighbour associated with that interface to a down state too. Any prefixes where the EIGRP neighbour was had the successor route will need to perform path recalculation to try find new routes.
The quickest method for an alternative route to be installed by EIGRP is if there is a feasible successor route. The feasible successor will instantly become the successor route, providing an alternative for the lost prefix.
The affected router will send an EIGRP update packet for that path due to the new EIGRP metrics now in play for that route, allowing neighbouring routers to update their own metrics for that path.
If there is not a feasible successor route, DUAL will need to perform a new route calculation. The route will change from a passive to active state in the EIGRP topology table.
The router will send out EIGRP query packets to neighbours regarding the lost route. The delay will be set to infinity in the packet to notify to other routers that the prefix has been set as active from passive.
When neighbouring routers receive the query packet, it may do one of the following:
- It may reply to the query that it does not have a route to the prefix
- If the query did not come from the successor of that route on the local router, it ignores it. Instead, it replies with the information of the EIGRP attributes for the route to the originating router.
- If the query did come from the successor of that route, the router detects that the delay is set to infinite, and sets the prefix as active in it’s own topology. The query packet process is repeated to notify all other EIGRP neighbours downstream
The query packet continues to make its way down the network until it reaches the boundary. The query boundary is when a router does not mark the prefix as active in it’s topology, either when scenarios one or two occur from above.
When the router has received a reply from every query packet that has been sent out, the DUAL is completed. The prefix is changed to passive and a reply packet is sent to any upstream routers that had sent a query packet to it.