RD, FD, FC, successor, feasible successor, feasibility condition.
1 - Feasible distance is the overall path metric (FD).
Below is an example of the path metric using the basic topology.
R1
R1#sho ip route eigrp | ex 10.3.4|10.2.4|10.0
2.0.0.0/32 is subnetted, 1 subnets
D 2.2.2.2 [90/130816] via 10.1.2.2, 01:11:58, GigabitEthernet1
3.0.0.0/32 is subnetted, 1 subnets
D 3.3.3.3 [90/130816] via 10.1.3.3, 00:07:20, GigabitEthernet2
4.0.0.0/32 is subnetted, 1 subnets
D 4.4.4.4 [90/131072] via 10.1.3.3, 00:03:26, GigabitEthernet2
[90/131072] via 10.1.2.2, 00:03:26, GigabitEthernet1
2 - Successor is the path with the lowest feasible distance (Best path).
R1 has two paths to reach network 10.2.4.0/24 but only one of them is in the topology table as successor.
As per the output, the successor for 10.2.4.0/24 is R2 as indicated by the next hop IP 10.1.2.2 with the FD of 3072.
R1 also has two paths to reach 4.4.4.4/32, but it has two successors in the topology table because the FDs are the same.3 - Feasible successor is the next best path (as long as it meets the feasibility condition). If the successor route fails, the Feasible successor path is moved to the routing table right away, without EIRGP recalculation.
The bandwidth from R1->R3->R4 have been changed to make the seconp path via R2 a feasible successor. The below example shows the output.
R1#sho ip eigrp topology | be 10.2.4 P 10.2.4.0/24, 1 successors, FD is 7680 via 10.1.2.2 (7680/5120), GigabitEthernet1 via 10.1.3.3 (7936/5376), GigabitEthernet2
4 - Feasibility condition states that as long as the backup path RD is less than the FD of the successor path, then we have a feasible successor.
Considering the path from R1 to 10.2.4.0/24, there are two physical paths but only one successor and no feasible successor.
- Example showing only the successor to 10.2.4.0/24 (sho ip eigrp topology)
R1#sho ip eigrp topology | be 10.2.4
P 10.2.4.0/24, 1 successors, FD is 3072 via 10.1.2.2 (3072/2816), GigabitEthernet1 P 2.2.2.2/32, 1 successors, FD is 130816 via 10.1.2.2 (130816/128256), GigabitEthernet1 P 10.3.4.0/24, 1 successors, FD is 3072
- Another example showing all the available paths. (sho ip eigrp topology all-links).
As we can see, a second path is displayed with RD of 3072 which is not less than the FD of the successor. Actually, the RD is equal to the FD of the successor, still since it is not less, it does not meet the feasibility condition.
R1#sho ip eigrp topology all-links | be 10.2.4
P 10.2.4.0/24, 1 successors, FD is 3072, serno 31
via 10.1.2.2 (3072/2816), GigabitEthernet1
via 10.1.3.3 (3328/3072), GigabitEthernet2
P 2.2.2.2/32, 1 successors, FD is 130816, serno 30
via 10.1.2.2 (130816/128256), GigabitEthernet1
As result of not having a feasible successor, if the best path to 10.2.4.0/24 fails, EIGRP need to recalculate to find another successor via queries. The same does not happen if successor and feasible successor exist already. EIGRP just install the feasible successor in the routing table.
5 - RD is the distance that the neighbor is reporting to us and is the neighbor FD.
In the below example we can see that from R1's perspective, there are two paths to 4.4.4.4, one via R2 another via R2. Both R2 and R3 are reporting the distance of 130816, which ends up to their FD to 4.4.4.4.
R1#sho ip eigrp topology P 4.4.4.4/32, 2 successors, FD is 131072 via 10.1.2.2 (131072/130816), GigabitEthernet1 via 10.1.3.3 (131072/130816), GigabitEthernet2
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