Multicast issue

[An123]

@Enes Simnica As u said: As for multicast packet amplification in SR-IOV, it’s a known bug where multicast packets get duplicated across multiple VFs. Even if MACs are unique, the switch sees the same traffic from different interfaces, which causes MAC flapping. This isn't a physical loop, but it looks like one to the switch. 

here all vms (vfs) are connected to single switch port where pf nic connected, then how switch sees the same traffic from different interfaces? This bug what do? for incoming and outgoing traffic..

[Enes Simnica]

@An123  let me dive a little bit more deeper here. And again, good question. So, Even though all the VFs (VMs) are connected through a single physical NIC and switch port (via the PF), the switch sees multiple logical MAC movements because of how SR-IOV handles traffic internally. Which means that when this multicast amplification bug kicks in, the NIC may replicate the same multicast frame across multiple VFs, and depending on the NIC driver/firmware behavior, these frames can be tagged or sourced inconsistently, causing the switch to think the MAC is moving or flapping, even though it’s technically coming from the same physical port.

To be more clear, to the switch it can appear like: 1. The same MAC is showing up from different logical paths, possibly even with slight changes in encapsulation or VLAN handling (especially with overlays like VXLAN involved). and then  updates its MAC table back and forth, thinking the MAC is unstable, which is what causes the flapping.

SO, The bug primarily affects outgoing multicast traffic from the host, where a single multicast packet gets replicated unintentionally across VFs, causing a flood of near-identical frames. The incoming traffic isn’t the trigger, it's mostly the egress (outbound) multicast behavior that breaks things.

hope it helps...

-Enes

[Enes Simnica]

and here u have some links, for Multicast issues, MAC issues and more........:

• https://www.cisco.com/c/en/us/support/docs/ip/ip-multicast/16450-mcastguide0.html
• https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol-stp-8021d/221722-troubleshoot-mac-flaps-loop-on-cisco-cat.html
• and this is a Cisco live called MULTICAST: THE USUAL SUSPECTS; https://www.ciscolive.com/c/dam/r/ciscolive/global-event/docs/2022/pdf/BRKENT-2264.pdf

and the best way to answer ur questions would be to lab them. Use EVEng or paste here some show commands if that is possible.......

[MHM Cisco World]

Are you config any port redundacy in VM?

Are you config any PO between VM and SW?

MHM