Please help with "show ip eigrp topology

rondan1955

One of the practice questions I've been working on asks "From the output shown, which routes will be in the routing table?"

I know that only successor routes appear in the routing table, but how can I tell the difference between successor and feasible successor? None of the cisco websites explain this clearly.
Any help would be greatly appreciated. And please don't just give me the answer, tell me how you got it.
Thanks!


R1# 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
! Lines omitted for brevity
! Below, the metric in the IP routing table entries match the first number in
! the parentheses, as well as the number listed as "FD is..." in the output above.

APA

rondan1955 wrote:

One of the practice questions I've been working on asks "From the output shown, which routes will be in the routing table?"

I know that only successor routes appear in the routing table, but how can I tell the difference between successor and feasible successor? None of the cisco websites explain this clearly.
Any help would be greatly appreciated. And please don't just give me the answer, tell me how you got it.
Thanks!


R1# 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
! Lines omitted for brevity
! Below, the metric in the IP routing table entries match the first number in
! the parentheses, as well as the number listed as "FD is..." in the output above.

The last route 172.31.24.0/30 is the only one with a feasible successor at the moment......

If you look at the two next hops

- via 172.31.11.2 (768/512), FastEthernet0/0
- 768 = Feasible Distance (The distance between the local router and the next hop router + the Advertised distance)
- 512 = Advertised Distance (The distance between the next hop router and the destination - 172.31.24.0/30)


- via 172.31.14.2 (1024/512), Serial0/0.4
- 1024 = Feasible Distance
- 512 = Advertised distance

For a route to be a feasible successor the advertised distance cannot be more than the feasbile distance of the current sucessor route. So as you can see the sucessors FD is 768 while the feasible successors AD = 512 which means the second route via 172.31.17.2 is a valid feasible successor. So upon losing the sucessor route the feasible successor can take over immediately without the need for the route to go into ACTIVE mode for recomputation.

As you can see from the topology table output there is only 1 successor for each route and next to that the FD is stated

- P 172.31.24.0/30, 1 successors, FD is 768
- The FD is the feasible distance as I highlighted in the explanation above, look at each available route and it's FD..... this is how you can tell which route is the current successor.

- To find out the valid feasbile successor if more than one is available have a look at the route with the next lowest FD. A route will not be a valid feasbile successor it it's AD is higher than the current successor's FD - this is to prevent routing loops

Hope this helps

P.S - remember eigrp can perform unequal-cost path load balancing if enabled

rondan1955

So according to your explanation,

D 172.31.211.0/24 via 172.31.11.201

and

D 172.31.24.0/30 via 172.31.11.2

will be in the routing table?

Also, am I correct in assuming that 172.31.151.0/24 will not show up in the routing table because it is directly connected?

Thanks for your help.

gojericho0

All directly connected routes will show up as well because they have a lower Administrative distance. The EIGRP rules above are just for alternative routes learned by neighbors. If the AD would be > the the FD of the successor, a potential routing loop could be created.

This is the big pitfall for distance vector routing protocols since they learn info from the neighbors. To compensate for this, during a failure EIGRP will query any of these routes that have a greater AD to see if the path is loop free.

You can view all possible paths by using the sh ip eigrp topology command