EIGRP: Feasible successor

nycbpnycbp Member Posts: 21 ■□□□□□□□□□
Hi guys,

I'm getting a bit confused about how a route can become a feasible successor. From what I've read numerous times, the rule is that the AD(advertised distance or reporting distance) of the route must be "LESS" than the FD(feasible distance) of the successor route.

The reason for my confusion is that I've taken some practice exams where they say the definition is "LESS THAN OR EQUAL TO" modifying what I originally wrote above.

R1# show ip eigrp topology
IP-EIGRP Topology Table for AS(1)/ID(
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status 
P, 1 successors, FD is 768
         via Connected, Loopback1
P, 1 successors, FD is 1024
        via (1024/760), FastEthernet0/0
P, 1 successors, FD is 768
        via (768/512), FastEthernet0/0
        via (1024/512), Serial0/0.4
! Lines omitted for brevity

From what I can see, only can become feasible successor since it's AD=512 which is less than the successor's FD=768.

Can anyone confirm this?


  • pham0329pham0329 Member Posts: 556
    AD/RD must be LESS than the FD to be a FS. The practice exam is wrong.
  • nycbpnycbp Member Posts: 21 ■□□□□□□□□□
  • thedramathedrama Member Posts: 291
    Thats right. Must be LESS than as pham said.

    Additionally you should care that feasible distance is related to successor. Reported distance is related to feasible successor. Candidates for best path are examined and lowest value between them selected successor. to choose feasible successor, that value is compared to feasible distance candidates. The first lower value than successor is elected feasible successor.
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