Successor vs FD routes.

[tony_bilton]

Hi all,

Todd makes a note on page 441 of the 5th edition about feasible successor routes. For a route to become a feasible successor, it's advertised distance must be less than the fesible successor..

1: Is the feasible successor the same as feasible distance as the former is the best route to the remote network.

2: If a route is to become a feasible successor with it's advertised distance is less than the successor route, why does that route not replace the successor route?

Furthermore, I'm using Todd's 5th edition book to study for the ccna. With all the updates will the information within this book be enough to pass along with practising the commands on my home kit?

[Big Evil]

If you have two routed to net a.
R1 via R2
or
R1 via R3

The path through has a cost of 75, through R3 it is 90.
R2 would thus be the successor.

For the path through R3 to be a feasible successor its FD must be lower than the AD to R2. Which it is as 40 is less than 50.

HTH.

Also, yes Todds book + hands on = Pass!

[Fuzz]

The terms used can get confusing, but it works like this.

A router will send a hello with a reported distance to a particular route. This is the cost to that destination from this router (its own FD for that route). Lets call this router A. Router B receives the hello, and looks at the reported distance. It then adds its own cost to the metric and creates its own FD for this route. If this is the lowest cost route, it becomes the successor route, and is placed into the routing table.

For a route to be a feasible successor, it's reported distance must be less then the FD of the successor route. This makes sure that the route does not go back through the same router and create a loop.

Use the IOS command sh eigrp topology to see the reported distances for each route, aswell as the FD for the lowest route (successor). You will see how different route metrics are used to create successors and feasible successors. Seriously, you need to see this output for yourself to really understand it.

[tony_bilton]

Thanks all for your help