EIGRP Feasible Successor routes

I always forget what these means in-between exams (I simply don't use EIGRP anywhere), but it is quite important to understand EIGRPs computation logic. The logic is this - when looking at possible 'backup' routes to hold ready for use, it's important to avoid the possibility of considering a path which has come from a neighbor which is going to send the packet back to you in the end - i.e. a routing loop. Without the 'global view' of link state protocols - you need to have a way to determine what inbound advertisements are 'safe' and which ones are 'maybe not safe'.

If you've a backup route which you know is safe, you can switch over really quickly in the event of the loss of your primary link. If you don't - you want to take a little time and be sure that you're not going to introduce a loop. You need to wait for the network to re-converge. If you've your head around that - the rest is just terminology.

First key term is 'Feasible Distance'. This is basically the metric of the current 'best' route (i.e. the live one), including any metric elements that were added by the local router. The 'whole cost' if you like. Let's make up a number and say it's 10,000.   'Reported Distance' of an inbound route advertisement is the metric that the neighbor is passing to us, before we add on any metric ourselves.

There are two possibilities for the Reported Distance, it's either more or less than our current 'best' metric - the Feasible Distance. If it's more, say 20,000, then it's possible that that neighbor is actually advertising our own advertisement back to us - i.e. a loop. Maybe it's not, but until the network has re-converged, we have to consider the possibility.  If the RD is less, then it is not possible that it's going back via us. This cannot be a loop, and therefore it's safe to quickly switch to this link.

Feasible Successors are quite simply an alternative path to the live route (the 'Successor'), which has a lower reported distance than the current feasible distance - i.e. backup routes which we know are not a loop. If a router looses it's main route, and has a feasible successor, it simply promotes it to the 'live' route, and sends out updates to it's neighbors to tell them the metric has changed.

If it doesn't have a feasible successor, then the router has no quick backup path, marks the route as 'active' (a counter intuitive tern which means the router is actively trying to work out how to get to the route in question), and starts querying it's neighbors to see if any of them know an way to get to the destination. Either it'll get an answer back with a loop free route, which will be installed as the new successor (i.e live route), or it'll give up and remove the route from the routing table..