Help with eigrp feasible successor routes

[kevingilmore]

I'm a bit confused and hoping someone can help.

In my EIGRP lab, R1 has two routes to the 192.168.3.0 network.

The fastest route (R1 to R3) is obviously the successor route.

But the 2nd route (R1 to R2 to R3) doesn't registered as a feasible successor, I understand that the rule is preventing this (in order for a route to be feasible the Advertised Distance of the alternate route must be lower than that of the Feasible distance of the current route).

Now, i change the bandwidth on s0/1 of R2 from 1544 (default T1) to 1543 and bam! it shows up as a feasible successor.

But why does lowering the bandwidth make the route more preferable, surely this makes the route less preferable because its got lower bandwidth and therfore slower?

Thanks

[John Blakley · Accepted solution]

Hi,

I'm not seeing the same thing you are by changing the bandwidth. Normally, the recommendation is to change delay instead of bandwidth since bandwidth affects other things such as qos and other routing protocols. You can change delay on the interface though. It's cumulative through the path.

I created a loopback on R3 and advertised that into eigrp. This interface had a delay of 5000 microseconds. R1 had seen a total delay of 25000 microseconds out interface via s0/1. It had a total of 45000 microseconds via R2, so it wasn't seen as a feasible successor. I changed the delay on R2 s0/1 to 50 (50 * 10 = 500 delay). This changed the path via R2 to 25500 and made it a feasible successor:

On R1 before the change:

R1(config-if)#do sh ip eigrp top 192.168.3.0/24

IP-EIGRP (AS 100): Topology entry for 192.168.3.0/24

  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2297856

  Routing Descriptor Blocks:

  172.160.13.3 (Serial0/1), from 172.160.13.3, Send flag is 0x0

      Composite metric is (2297856/128256), Route is Internal

      Vector metric:

        Minimum bandwidth is 1544 Kbit

        Total delay is 25000 microseconds

        Reliability is 255/255

        Load is 1/255

        Minimum MTU is 1500

        Hop count is 1

  172.160.12.2 (Serial0/0), from 172.160.12.2, Send flag is 0x0

      Composite metric is (2809856/2297856), Route is Internal

      Vector metric:

        Minimum bandwidth is 1544 Kbit

        Total delay is 45000 microseconds

        Reliability is 255/255

        Load is 1/255

        Minimum MTU is 1500

        Hop count is 2

P 192.168.3.0/24, 1 successors, FD is 2297856

        via 172.160.13.3 (2297856/128256), Serial0/1

R1(config-if)#

On R2:

R2(config)#int s0/1

R2(config-if)#delay 50

R2(config-if)#

Back on R1:

P 192.168.3.0/24, 1 successors, FD is 2297856

        via 172.160.13.3 (2297856/128256), Serial0/1

        via 172.160.12.2 (2310656/1798656), Serial0/0

Notice the change:

172.160.12.2 (Serial0/0), from 172.160.12.2, Send flag is 0x0

      Composite metric is (2310656/1798656), Route is Internal

      Vector metric:

        Minimum bandwidth is 1544 Kbit

       Total delay is 25500 microseconds

        Reliability is 255/255

        Load is 1/255

        Minimum MTU is 1500

        Hop count is 2

HTH,

John

*** Please rate all useful posts ***

[dslice · Accepted solution]

Provide output before and after the change. Show ip eigrp topo network mask

Sent from Cisco Technical Support iPad App