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OSPF and ISIS flapping with reason 'InActiveTimer'

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Article ID: KB37308 KB Last Updated: 13 Sep 2021Version: 1.0
Summary:

This article explains why OSPF/ISIS flaps with reason 'InactiveTimer' and the possible scenarios in which this can happen.  A troubleshooting command is provided to quickly narrow down whether the issue is with RPD utilization and/or host queue on the device.

Symptoms:

OSPF and ISIS may flap with the following message:

Jun  1 07:06:24  router-name rpd[6987]: %DAEMON-5-RPD_OSPF_NBRDOWN: OSPF neighbor 100.64.1.1 (realm ospf-v2 ae157.0 area 0.0.0.0) state changed from Full to Down due to InActiveTimer (event reason: neighbor was inactive and declared dead)
Cause:
  1. This is an indication that the consecutive hellos are not received/seen by the local router and the hold time has expired for that particular adjacency/neighbor.

  2. This can happen when the neighbor is indirectly connected to the local router and either the peer is down or there is an indirect link failure.

  3. There is also a possibility the local router received a burst of other control packets at a point where it is not able to handle them, which leads to dropping subsequent hellos for ISIS/OSPF and eventually dropping ISIS/OSPF adjacency.

  4. Another possibility is RPD utilization consistently stays high and ISIS/OSPF protocols are not getting enough cycles to process the hello packets. Then eventually ISIS/OSPF times out.

Solution:
  1. Check if there are any drops in the net.isr queue with the following sysctl command executed under RE shell:

    % sysctl net.isr.drop

    If drops are seen here,  it indicates either RPD daemon on this device is too busy or the host-path is getting congested.  Continue with the the following troubleshooting steps to narrow down cause.
  2. Check CPU/RPD utilization to see if it spiked to 100% while the ISIS/OSPF adjacency timed out.  For steps to troubleshoot high CPU, refer to the following articles:

  3. Collect the following DDOS output to see if there was any DDOS violation which could have lead to host path being congested.  Troubleshoot based on which protocol is violated.

    >show ddos-protection protocols
    For example, refer to KB35624 - Syslog message - DDOS_PROTOCOL_VIOLATION_SET: Host-bound traffic for protocol/exception exceptions:mtu-exceeded
  4. Check protocol traceoptions for any issues with the hello packets being missed:

    set protocols ospf traceoptions flag all 
    set protocols isis traceoptions flag all

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