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I have 2 links to a destination and EIGRP was setup. The faster link is the successor and the slower link is the feasible successor.

Switch1#show ip eigrp topology  192.168.1.0
EIGRP-IPv4 Topology Entry for AS(100)/ID(192.168.14.18) for 192.168.1.0/24
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 3072
  Descriptor Blocks:
  192.168.14.17 (GigabitEthernet1/0/10), from 192.168.14.17, Send flag is 0x0
      Composite metric is (3072/2816), route is Internal
      Vector metric:
        Minimum bandwidth is 1000000 Kbit
        Total delay is 20 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 1
        Originating router is 192.168.14.10
  192.168.14.2 (GigabitEthernet2/0/13), from 192.168.14.2, Send flag is 0x0
      Composite metric is (28416/2816), route is Internal
      Vector metric:
        Minimum bandwidth is 100000 Kbit
        Total delay is 110 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 1
        Originating router is 192.168.14.10

q1) Can I confirm, if "holdtime" for the successor's neighbour is over, the feasible successor will automatically become successor ?

q2) I encounter an incident yesterday whereby the successor went down for awhile but I am not sure if the feasible successor did actually take over. When I login to check, the successor has already recover.

All I see in the logging was the holding time expiry for the successor

001275: Sep 25 02:09:08: %DUAL-5-NBRCHANGE: EIGRP-IPv4 100: Neighbor 192.168.14.17 (GigabitEthernet1/0/10) is down: holding time expired
001276: Sep 25 02:18:35: %DUAL-5-NBRCHANGE: EIGRP-IPv4 100: Neighbor 192.168.14.17 (GigabitEthernet1/0/10) is up: new adjacency

I have no idea if the feasible successor did actually become the successor (even if it is just for awhile).

When a feasible successor become a successor , will there by any log generated ?

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There is a command made for such checking. Use:

show ip eigrp events

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  • Almost 20 years in networking, and I still learn something new every day. Thanks for that! – Marc 'netztier' Luethi Sep 25 '17 at 15:38
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  1. Yes, you can confirm that. Once hold-time expires, the former feasible successor route immediately becomes the successor route and is installed in routing table.
  2. I do not think we have a log message of when a feasible successor route becomes a successor route, given the fact that it happens right at the same time when the message of Neighbor 192.168.14.17 (GigabitEthernet1/0/10) is down: holding time expired generated.

The best way is to request a network downtime (non-production or off-business hours) and test your network with different cases. Note that EIGRP provides very fast convergence.

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  • 1
    To add to Hung Tran's answer: You can see what is being removed from or added to the routing table by looking at the output of debug ip routing in a second VTY (SSH/Telnet Session to the router), by virtue of logging monitor debugging & terminal monitor. Depending on your (dynamic) routing setup, a few dozen routes being popped and pushed might generate 100s of lines of debug output, so don't let this go to the console (by logging console <level> , where level is equal to or higher than informational) – Marc 'netztier' Luethi Sep 25 '17 at 13:21
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    And to add some more: debugging - by its nature - is not meant to be run during production workloads. Depending on system hardware and architecture, there can be significant impact on the device, up to unpleasant events like %SYS-3-LOGGER_FLUSHED: System was paused for [time-stamp] to ensure console debugging output - which is why you want to make sure that debugging output never goes to the console, but to "monitor". – Marc 'netztier' Luethi Sep 25 '17 at 13:32

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