this example will successfully explain when a feasible successor route will be introduced in
show ip route:
|---5-----R2--10--R3--10--|
R1- -switch---->R5----n/w
|----5-----R4----10----------|
Route 1> R1-->R4--->R5-->n/w
FD=5+10+5=20
AD=10+5=15
Route 2> R1-->R2-->R3-->R5-->n/w
FD=5+10+10+5=30
AD=10+10+5=25
Cost of Route 1 < Route 2 Hence Route 1 will be successor. Now we know that to qualify a route as a Feasible successor we need to have the AD of a feasible successor should be less than FD of a successor
so we need to compare the Route 2 AD and Route 1 FD. But we found that 25 is not less than 20. Hence there will be no feasible successor.
Hope this helps
I know that only successor routes appear in the routing table, but how can I tell the difference between successor and feasible successor?
ReplyDeleteR1# show ip eigrp topology
IP-EIGRP Topology Table for AS(1)/ID(172.31.16.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 172.31.151.0/24, 1 successors, FD is 768
via Connected, Loopback1
P 172.31.211.0/24, 1 successors, FD is 1024
via 172.31.11.201 (1024/76, FastEthernet0/0
P 172.31.24.0/30, 1 successors, FD is 768
via 172.31.11.2 (768/512), FastEthernet0/0
via 172.31.14.2 (1024/512), Serial0/0.4
======================================
P 172.31.24.0/30, 1 successors, FD is 768
via 172.31.11.2 (FD768/AD512), FastEthernet0/0
via 172.31.14.2 (FD1024/AD512), Serial0/0.4
since 14,2's AD< FD of 11.2, it will be added as feasible successor