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ELRP does not detect a loop

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TitleELRP does not detect a loop
Symptoms
- Broadcast storm in the network due to an untagged loop.
Environment
EXOS switches, other switches may be involved as well.
Cause
ELRP uses a layer-2 multicast packet to detect loops in a network. The switch sends an ELRP frame out of a port in a configured VLAN, and if the switch receives that same packet back, in the same VLAN, then the switch takes the configured action for ELRP.

If the packet arrives to the switch in a different vlan, the switch will drop the packet.
Resolution
  1. On the first switch, run a
    run script spath-stats.py
    This script is embedded in EXOS and will display counts of packets going to the CPU by port and VLAN. 
  2. If spath-stats.py isn't available on your switch, run the command
    clear l2stats
    and then
    show l2stats
    several seconds later. This will give you a count, by VLAN, on the number of packets going to the CPU. Look for high numbers in each of the VLANs.
  3. show ports utilization bandwidth port-number
    can also help determine the ports with high utilization.
  4. Follow the path of the ports connected on that vlan and look for untagged differences. For instance, we are looking for "show port <port> information detail" Internal tag Y connected to "show port <port> information detail" Internal tag Z.
  5. Change the Z tag to match Y in order to fix the untagged loop. At that point, ELRP should find the loop. If it doesn't, there may be more misconfigurations.

     
Additional notes

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