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Cisco Bug: CSCvh93806 - [2960x] flexlink multicast fast convergence leaking IGMPv3/v2 Reports causing loop

Last Modified

Sep 17, 2019

Products (1)

  • Cisco Catalyst 2960-X Series Switches

Known Affected Releases

15.2(2.0.0)

Description (partial)

Symptom:
Cat2960x stack connecting to two upstream devices in triangle topology
Cat2960x stack is configured with FlexLink multicast fast convergence feature
This is designed to send IGMP Reports via both Active and Backup interface to upstream devices to speed up multicast convergence in case of a link failure
Upstream device  is IGMP Querier however it is running IGMPv2 by default
Upon reception of IGMPv3 Report which is destined to 224.0.0.22 // 0100.5e00.0016 this is treated as regular multicast and hence flooded out via all interfaces within the VLAN apart from the one it was received on
As a result, both upstream devices receive this IGMPv3 Report [one from Active FlexLink port and the other from Backup FlexLink port] and both flood it to each other
As a result IGMPv3 Reports are looping in this triangle topology

This is impacting any other control plane protocols that are enqueued in the same CPU Queue as IGMP traffic [multicast control]
If platform does have CPU Queue rate limiters, CPU utlization will not be 100% but control plane traffic will be dropped by policer
If platform does not have CPU Queue rate limiters, CPU utlization will be 100% and control plane traffic could be dropped due to high CPU

Conditions:
Cat2960X with FlexLink configured with multicast fast-convergence option
IGMPv3 reports present on vlan allowed on FlexLink pair
issue seen in all latest software [15.2(2)E8 / 15.2(4)E5 / 15.2(6)E]
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