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EIGRP FD and FS Calculation and Semantics

kfarrington
Level 3
Level 3

Guys,

Just a couple of question in regards to EIGRP entries in the topology table.

One :-

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Would it be correct to say that there is only ONE FD per prefix in the topology table. Ie, If I see the following output from the "show ip eigrp topo all" entries, the "ONE FD for the prefix" FD is 3072 but on the individual lines, each numeric value before the "/" is not the FD, this is just the total metric to the destination network and the entry after the "/" is the RD.

RTR1>sh ip eigrp top all

IP-EIGRP Topology Table for AS(121)/ID(155.195.58.44)

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

r - reply Status, s - sia Status

P 155.195.44.64/26, 3 successors, FD is 3072, serno 478092

via 155.195.45.93 (3072/2816), Vlan455

via 155.195.45.94 (3072/2816), Vlan455

via 155.195.45.95 (3072/2816), Vlan455

via 155.195.45.91 (28416/28160), Vlan455

via 155.195.45.86 (28416/28160), Vlan455

via 155.195.47.13 (3328/3072), Vlan302

via 155.195.47.5 (3328/3072), Vlan202

via 155.195.45.81 (28416/28160), Vlan455

via 155.195.45.79 (28416/28160), Vlan455

via 155.195.45.75 (28416/28160), Vlan455

via 155.195.45.71 (28416/28160), Vlan455

via 155.195.45.76 (28416/28160), Vlan455

via 155.195.45.92 (28416/28160), Vlan455

via 155.195.45.82 (28416/28160), Vlan455

Ie, in the output above, we have ONE FD value of 3072, and the other values of 28416, 3328 you cannot call these feasible distances and would just call them metrics to the destination.

OR is is better to use the term there is only one ACTIVE FD per prefix and all the other entries above, ie 28416 and 3328 are FDs for that particular neighbor?

Two:

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Please see the diagrams attached for the age old question about RD == FD and this is not an FS.

Firstly on the diagram "EIGRP Topology- No Feasible Successor.jpg" as FD == RD, there is no FS and thus when the successor link is brought down, EIGRP queries for a new path and this is then indicated by the fact a NEW FD is calculated for the prefix as is seen in the topology table.

Secondly, on the diagram "EIGRP Topology- With a Feasible Successor.jpg" you see that when the FS takes over (no EIGRP query) in the topology table the FD does NOT change but the routing table metric does.

So the clarification here is that is it safe to assume when an FS takes over from a successor, you can tell this in the output of the eigrp topology table as the prefix metric will be HIGHER than the feasible distance and the reported distance will be LOWER than the FD. This I think is an important statement to clarify.

Three:

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In regards to question two, If a prefix successors route goes down and there is a feasible successor, as mentioned, the FD for the prefix will NOT change. But then say that the successor does not come back into operation, say a decommissioned link, so the FS (which is now the S) is now the preferred path and will remain that way for ever. Is there a timer value for DUAL to recalculate the route - realigning a new FD for the prefix in the topology table and finding any new FSs than now align with the new FD.

I hope this does make sense and please let me know if anyone needs further clarification.

Kind regards,

Ken

16 Replies 16

more results - mess 2 of 2

Great stuff, Ken.

The lab results do indeed confirm out theoretical understanding of how this works.

Regards,

Paresh.

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