Virtual Machine Fails to Receive Multicast Traffic Despite Successful Group Subscription

Products

• VMware vCenter Server

Issue/Introduction

• A monitoring server reports that a virtual machine (VM) configured in a multicast group is failing to receive multicast data.
• Packet captures (PCAPs) on the affected VM confirm that it is successfully broadcasting IGMP join requests to subscribe to the multicast group.
  
• Despite the successful subscription, the VM receives no incoming multicast data packets (UDP traffic).

Environment

VMware vCenter

Cause

Packet captures on the ESXi host's physical network interface (vmnic) reveal that multicast UDP traffic is not reaching the host from the upstream network. Because the physical switches are not forwarding the traffic to the host, the virtual switch cannot deliver it to the destination VM. The issue is isolated to the physical network infrastructure, not the VMware virtual environment.

Resolution

Because the traffic drop occurs before reaching the virtual environment, the issue must be resolved at the physical network layer.
Engage your physical network infrastructure team to perform the following troubleshooting steps:

• Trace the multicast path: Track the multicast data packets through the upstream network to locate the exact drop point.
• Verify IGMP Snooping/Querier: Ensure that IGMP snooping is properly configured on the physical switches and that an active IGMP querier is maintaining the group states.
• Check Routing & ACLs: Verify Protocol Independent Multicast (PIM) configurations on upstream routers and ensure no Access Control Lists (ACLs) or firewalls are blocking the UDP multicast stream.