How is FD calculated?

Answered Question
Apr 13th, 2007
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Quoted from the bible we have "The lowest calculated metric to each destination will become the feasible distance (FD) of that destination." It is simple and clear enough. But today I found that it looks like the router does not follow this well when calculate the FD:


Router#sh ip eig top 194.62.172.16 255.255.255.240

IP-EIGRP (AS 66): Topology entry for 194.62.172.16/28

State is Passive, Query origin flag is 1, 0 Successor(s), FD is 4294967295

Routing Descriptor Blocks:

10.252.2.97 (Vlan954), from 10.252.2.97, Send flag is 0x0

Composite metric is (308224/307968), Route is External

Vector metric:

Minimum bandwidth is 10000 Kbit

Total delay is 2040 microseconds

Reliability is 255/255

Load is 9/255

Minimum MTU is 1500

Hop count is 5

External data:

Originating router is 30.249.51.6

AS number of route is 69

External protocol is EIGRP, external metric is 307968

Administrator tag is 69 (0x00000045)

10.252.2.1 (Vlan951), from 10.252.2.1, Send flag is 0x0

Composite metric is (308224/307968), Route is External

Vector metric:

Minimum bandwidth is 10000 Kbit

Total delay is 2040 microseconds

Reliability is 255/255

Load is 6/255

Minimum MTU is 1500

Hop count is 5

External data:

Originating router is 30.249.1.6

AS number of route is 69

External protocol is EIGRP, external metric is 307968

Administrator tag is 69 (0x00000045)

IP-EIGRP (AS 69): Topology entry for 194.62.172.16/28

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 283392

Routing Descriptor Blocks:

141.228.112.194 (GigabitEthernet1/22), from 141.228.112.194, Send flag is 0x0

Composite metric is (283392/283136), Route is External

Vector metric:

Minimum bandwidth is 10000 Kbit

Total delay is 1070 microseconds

Reliability is 255/255

Load is 1/255

Minimum MTU is 1500

Hop count is 7

External data:

Originating router is 30.249.2.6

AS number of route is 66

External protocol is EIGRP, external metric is 282880

Administrator tag is 66 (0x00000042)

Router#


We can see that for AS 66, router receives advertisement from 2 neighbours, and their calculated metrics to the destination are both 308224. Thus by definiation, the FD should be 308224 as well. But strangely we can see that the show FD is shown as 4294967295 instead. May someone explain me where does the wierd number comes from? Thanks.

Correct Answer by bmbreer about 10 years 2 weeks ago

4294967295 is the largest number you can represent with 32 bits. This means that the feasible distance is inaccessible. This typically means the route is overridden by another routing protocol that has a lower administrative distance.


Check your routing table to see what routing protocol is the source of the 194.62.172.16/28 entry. Are you doing any route redistribution? You may need to change the routing protocol's administrative distance so that external eigrp routes are preferred.

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Harold Ritter Fri, 04/13/2007 - 12:42
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The FD is set to inaccessible so that we do not propagate this route to our neighbors. It is probably due to 194.62.172.16/28 already being installed in the RIB with a better admin distance.


Hope this helps,

royalblues Fri, 04/13/2007 - 13:09
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Friend,


As Harold said ealier, EIGRP reports an unreachable destination by a delay of 4294967295 which is 0xFFFFFFFF in Hex.


This cna also happen when there is no route to the destination and the route had queried its neighbors for an alternate route to the destination .

Since the route is passive now, none of the neighbor has replied with an alternate path for the queries


HTH, rate if it does

Narayan


Correct Answer
bmbreer Fri, 04/13/2007 - 13:02
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4294967295 is the largest number you can represent with 32 bits. This means that the feasible distance is inaccessible. This typically means the route is overridden by another routing protocol that has a lower administrative distance.


Check your routing table to see what routing protocol is the source of the 194.62.172.16/28 entry. Are you doing any route redistribution? You may need to change the routing protocol's administrative distance so that external eigrp routes are preferred.

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