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When VPC operating as STP Edge port is being flapped, we observe TCN trigger in the IGMP events, resulting in temporary multicast drop
1. Observed on N3k and N9k switches. 2. STP Edge configuration on the VPC port-channel. 3. VPC port-channel being flapped. 4. IGMP snooping must be enabled. 5. Multicast receivers connected to the VPC switches on the affected VLANs.
No workaround available.
When flapping VPC port-channel, the IGMP process is detecting STP TCN and triggers IGMP General leave packet. That is not expected for Edge port that it's not generating STP TCNs when it's being flapped. N9K-C93180YC-1# show clock 19:35:54.077 UTC Mon Nov 08 2021 N9K-C93180YC-1(config)# int po 2 N9K-C93180YC-1(config-if)# shut N9K-C93180YC-1(config-if)# no shut N9K-C93180YC-1(config-if)# show ip igmp snooping event-history vlan vlan Events for IGMP Snoop process 2021 Nov 8 19:36:04.239250 igmp [25775]: [26035]: SN: Triggering global-leave to router port Po1 2021 Nov 8 19:36:04.239229 igmp [25775]: [26035]: SN: Triggering IGMP snoop general leave due to STP TCN for vlan 10 2021 Nov 8 19:35:58.443068 igmp [25775]: [26035]: SN: igmp_snoop_process_interface_deletion: adminshut 2021 Nov 8 19:35:58.443038 igmp [25775]: [26035]: SN: Processing interface admin-shut notification for Po2 (8) The STP TCNs did not change: VLAN0010 is executing the rstp compatible Spanning Tree protocol Number of topology changes 2 last change occurred 0:19:39 ago from port-channel1 The port-channel is configured to be Edge and it’s operating that way: Port 4097 (port-channel2, vPC) of VLAN0010 is designated forwarding Port path cost 1, Port priority 128, Port Identifier 128.4097 Designated root has priority 32778, address 00be.755b.dbd7 Designated bridge has priority 32778, address 00be.755b.dbd7 Designated port id is 128.4097, designated path cost 0 Timers: message age 0, forward delay 0, hold 0 Number of transitions to forwarding state: 3 The port type is edge Link type is point-to-point by default BPDU: sent 133, received 0 During that time, we can observe IGMP leave packets in Ethanalyzer on the VPC peer: N9K-C93180YC-2# ethanalyzer local interface inband display-filter igmp limit-captured-frames 0 Capturing on inband 2021-11-08 18:20:48.887770 0.0.0.0 -> 224.0.0.2 IGMP V2 Leave Group, general 2021-11-08 18:20:48.889429 0.0.0.0 -> 224.0.0.2 IGMP V2 Leave Group, general N9K-C93180YC-2# show ip igmp snooping event-history vlan vlan Events for IGMP Snoop process 2021 Nov 8 19:36:08.291332 igmp [25759]: [26077]: SN: Forwarding to internal SVI Vlan10 () 2021 Nov 8 19:36:08.291323 igmp [25759]: [26077]: SN: Don't forward back on router-port Po1 . 2021 Nov 8 19:36:08.291318 igmp [25759]: [26077]: SN: From vPC member link, dont send it on peer-link 2021 Nov 8 19:36:08.291300 igmp [25759]: [26077]: SN: Forwarding the packet to router-ports . (iif Po1) 2021 Nov 8 19:36:08.291290 igmp [25759]: [26077]: SN: While trying to forward packet should_check_report_flooding is true 2021 Nov 8 19:36:08.291279 igmp [25759]: [26077]: SN: Received a v2 leave on Po1 for group 0.0.0.0 2021 Nov 8 19:36:08.291243 igmp [25759]: [26077]: SN: Process a valid IGMP packet, pkttype:leave(23), iif:Po1
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