Eigrp Part 5: Topology Table & Fail over scenario.

Show ip EIGRP topology = This shows you the current successor and feasible successor routes in the topology table.

Show ip EIGRP topology all-links = This shows you the current successor and feasible successor routes plus any additional routes that can be used to reach specific prefixes (Subnets). 

Examples are below.

 If you look at subnet 172.16.2.0/24, you see a successor and feasible successor in the topology table but there is another path in my network to reach this route but it’s not considered a successor or feasible successor, it won’t show up in the topology table. If you use show ip EIGRP topology all-links you can see the additional route which will be used if the successor and feasible successor were to go down.

R2#show ip EIGRP topology
IP-EIGRP Topology Table for AS(10)/ID(192.168.1.2)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 10.10.10.0/30, 1 successors, FD is 28160
        via Connected, FastEthernet0/1
P 11.11.11.0/24, 2 successors, FD is 30720
        via 10.10.10.1 (30720/28160), FastEthernet0/1
        via 12.12.12.2 (30720/28160), FastEthernet1/0
        via 192.168.1.1 (1737216/28160), FastEthernet0/0
P 12.12.12.0/24, 1 successors, FD is 28160
        via Connected, FastEthernet1/0
P 192.168.1.0/24, 1 successors, FD is 1734656
        via Connected, FastEthernet0/0
        via 12.12.12.2 (337920/335360), FastEthernet1/0, serno 11
        via 10.10.10.1 (335360/332800), FastEthernet0/1
P 172.16.1.0/24, 1 successors, FD is 156160
        via 10.10.10.1 (156160/153600), FastEthernet0/1
        via 192.168.1.1 (1862656/153600), FastEthernet0/0
P 172.16.2.0/24, 1 successors, FD is 156160
        via 10.10.10.1 (156160/128256), FastEthernet0/1
        via 192.168.1.1 (1862666/128266), FastEthernet0/0
R2#

R2#show ip EIGRP topology all-links
IP-EIGRP Topology Table for AS(10)/ID(192.168.1.2)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 10.10.10.0/30, 1 successors, FD is 28160, serno 1
        via Connected, FastEthernet0/1
        via 192.168.1.1 (1734912/256256), FastEthernet0/0, serno 10
P 11.11.11.0/24, 2 successors, FD is 30720, serno 7
        via 10.10.10.1 (30720/28160), FastEthernet0/1
        via 12.12.12.2 (30720/28160), FastEthernet1/0
        via 192.168.1.1 (1737216/28160), FastEthernet0/0
P 12.12.12.0/24, 1 successors, FD is 28160, serno 2
        via Connected, FastEthernet1/0
        via 10.10.10.1 (33280/30720), FastEthernet0/1
        via 192.168.1.1 (1739776/30720), FastEthernet0/0
P 192.168.1.0/24, 1 successors, FD is 1734656, serno 3
        via Connected, FastEthernet0/0
        via 12.12.12.2 (337920/335360), FastEthernet1/0, serno 11
        via 10.10.10.1 (335360/332800), FastEthernet0/1
P 172.16.1.0/24, 1 successors, FD is 156160, serno 8
        via 10.10.10.1 (156160/153600), FastEthernet0/1
        via 12.12.12.2 (158720/156160), FastEthernet1/0
        via 192.168.1.1 (1862656/153600), FastEthernet0/0
P 172.16.2.0/24, 1 successors, FD is 156160, serno 9
        via 10.10.10.1 (156160/128256), FastEthernet0/1
        via 12.12.12.2 (158720/156160), FastEthernet1/0        via 192.168.1.1 (1862666/128266), FastEthernet0/0
R2#

While doing some EIGRP testing, I notice that all your EIGRP neighbors should have the same hold-time and hello interval configured. Imagine this, your successor path has a hello interval of 2 and hold-time of 6, should that path fail your feasible successor path will pickup in 6 seconds, but on your feasible successor path you never changed the default hello (5sec) and hold-time (15sec) interval which is fine for a single failure scenario.

What happens if you have a double failure scenario? You now have to wait 15 seconds for any convergence to happen since you never changed the timer on the feasible successor path. I tested this myself. Best practice is to have a baseline across your network and always test a double failure scenario.