Output of : sh ip eigrp topology

May 26th, 2007

I was doing simulations with Boson Software and when i did a "sh ip eigrp topology", i've got this :

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,

P 160.10.0.0/8, 1 successors, FD is 28160

via Summary(28160/0), Null0

P 160.10.0.2/0, 1 successors, FD is 28160

via Connected, FastEthernet0/0

P 175.10.0.0/8, 1 successors, FD is 28160

via Summary(28160/0), Null0

P 175.10.0.2/0, 1 successors, FD is 28160

via Connected, Serial0

My question is what does the "FD is 28160" stands for ?

Thx, Farell

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Replies

ankbhasi Sat, 05/26/2007 - 06:33

Hi Farell,

FD stands for Feasible Distance. The feasible distance is the best metric to reach the destination or the best metric that was known when the route went active.

This value is used in the feasibility condition check. If the reported distance of the router (the metric after the slash) is less than the feasible distance, the feasibility condition is met and that path is a feasible successor. Once the software determines it has a feasible successor, it need not send a query for that destination.

HTH

Ankur

blatkinson Wed, 06/20/2007 - 20:45

Regarding the feasibility condition check, can someone please provide an example of a situation in which the Reported Distance is MORE than the Feasible Distance?

mheusing Thu, 06/21/2007 - 09:23

Hi,

the whole concept gets important, if more than one neighbor exists. Assume Ra connected to a network N through two neighbors Rb and Rc.

Avertised distance (AD)of Rb to reach network N: 2500 and AD of Rc: 1500

What is the picture of Ra under those conditions? Well Ra knows that there are two pathes to N, but which one is the best? To find this out Ra has to do a little calculation: find the FD = minimum of AD+(metric to reach a neighbor) of all neighbors.

So if the metric to reach Rb is 2000 and the metric to reach Rc is 750 the conclusion of Ra is: FD=2250 (750+1500)

So far so good, we found the best path for Ra to N, can create a routing table entry and forward packets.

Still one part is missing. Does Ra have a feasible successor? The idea of a feasible successor is important in the backup case. Is Ra allowed to imediately reroute traffic for N through Rb if the link to Rc goes down?

In the above example it might seem natural, but there is one issue: what is the routing table entry in Rb? Actually Ra does not know. All it knows is it is closer to the destination (FD=2250) than Rb (AD 2500). So Ra cannot rule out that Rb has chosen Ra as next hop to reach N. Thus we would create a routing loop when immediately switching over to Rb.

So the requirement is ONLY if the FD for a network is worse than the AD of a neighbor this neighbor can become Feasible Successor.

Only then Ra can be sure the neighbor is not routing through Ra.

In the example above, advertised or reported distance of Rb is MORE than the FD of Ra.

If only one link to a distant network exists, this will never be the case.

Hope this helps!

Regards, Martin