EIGRP -- Basic concept - feasible successor

Answered Question
Jul 29th, 2008

In the following extract from a sh ip eigrp top all-links IS gi0/1 a feasible successor ? - The reported distance is equal to, rather than lower the FD - if fa0/22 fails does routing continue without a reconvergence

P 10.172.164.0/24, 1 successors, FD is 290304, serno 2524112

via 172.16.20.14 (290304/287744), FastEthernet0/22

via 172.16.22.25 (290560/290304), GigabitEthernet0/1

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Correct Answer by Giuseppe Larosa about 8 years 5 months ago

Hello Jonathan,

first of all I would consider if the EIGRP neighbor 172.16.22.25 can be a feasible successor for IP prefix 10.172.164.0/24 rather then looking at the possible outgoing interface

The answer is no

A feasible successor must advertise to the local router an EIGRP metric(AD advertised distance) less then the current FD (best metric on local router)

this is the feasibility condition of EIGRP dual

in this case AD = FD but from the point of view of the local router is not enough because using that AD will not be safe in order to avoid routing loops.

Hope to help

Giuseppe

Correct Answer by gojericho0 about 8 years 5 months ago

G0/1 would not be a feasible successor because the AD == the FD of the successor. In order for it to be feasible the AD must be less than the FD in order to help assure there is no loop.

In this case the router will perform DUAL queries to see if its neighbor (172.16.22.25) can indeed reach 10.172.164.0. Based on 172.16.22.25 AD it does not look like the 10 network is directly connected. Therefore it will keep querying its neighbors. If there is a loop, the query will eventually time out

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Correct Answer
gojericho0 Tue, 07/29/2008 - 07:12

G0/1 would not be a feasible successor because the AD == the FD of the successor. In order for it to be feasible the AD must be less than the FD in order to help assure there is no loop.

In this case the router will perform DUAL queries to see if its neighbor (172.16.22.25) can indeed reach 10.172.164.0. Based on 172.16.22.25 AD it does not look like the 10 network is directly connected. Therefore it will keep querying its neighbors. If there is a loop, the query will eventually time out

Correct Answer
Giuseppe Larosa Tue, 07/29/2008 - 07:14

Hello Jonathan,

first of all I would consider if the EIGRP neighbor 172.16.22.25 can be a feasible successor for IP prefix 10.172.164.0/24 rather then looking at the possible outgoing interface

The answer is no

A feasible successor must advertise to the local router an EIGRP metric(AD advertised distance) less then the current FD (best metric on local router)

this is the feasibility condition of EIGRP dual

in this case AD = FD but from the point of view of the local router is not enough because using that AD will not be safe in order to avoid routing loops.

Hope to help

Giuseppe

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