Symptom
IGMP Snooping entries learned from a vPC peer through Cisco Fabric Services (CFS) will repeatedly expire in approximately 3 x IGMP Query Interval time and will be relearned again within the next IGMP Query Interval time. The time to expire the CFS-learned IGMP Snooping entries (the duration of "3 x IGMP Query Interval") will not be longer than approximately 20 seconds. Also, exactly one arbitrary CFS-learned IGMP Snooping entry will not be affected by this defect.
Conditions
1.) The Nexus 5000 series switches are configured in a vPC domain as vPC peers.
2.) The IGMP Querier is an external device and the vPC peers are not acting as IGMP Queriers themselves.
3.) The IGMP Query Interval is less than 8 seconds.
Workaround
Configure the IGMP Query Interval to be at least 8 seconds.
Further Problem Description
When a switch running IGMP Snooping in a vPC domain first learns about a new multicast group by receiving an IGMP Membership Report, it records the multicast group and the port on which the Membership Report was received, and forwards this Membership Report over the CFS to the vPC peer. At the same time, the switch starts a non-configurable, 20-second per-group timer, a so-called mct_dup_oifl_report-timer (can be displayed in "show ip igmp snooping group X.X.X.X detail"). If another Membership Report is received for the same group on the same port while this timer is running, the switch will process the Membership Report locally but it will not forward this Membership Report over the CFS to the vPC peer. Because IGMP Snooping entries expire after a period of 3 x Query Interval seconds, then if this period is shorter than 20 seconds, the vPC peer will repeatedly expire and relearn the IGMP Snooping entries.
