Duplicate multicast packets received on a Distributed Port Group (DvPG) with Promiscuous Mode and ReversePathFwdCheckPromisc enabled

Products

• VMware vCenter Server

Issue/Introduction

Virtual Machines (VMs) connected to the same DvPG and exchanging link-local multicast traffic are receiving duplicate packets. These duplicate packets persist even when the /Net/ReversePathFwdCheckPromisc advanced setting is enabled on the ESXi host alongside Promiscuous Mode.

 

Environment

VMware ESXi

Cause

The ReversePathFwdCheckPromisc setting filters out packets (at the vNIC/switch port IOChain level) received on an uplink (vmnic) that is not mapped to the receiving switch port.

However, duplicate copies that arrive via the uplink associated with the receiving switch port will not be filtered by this mechanism. This can result in duplicate packets, especially with link-local multicast traffic, because it cannot leverage IGMP snooping. If no filtering is configured on the physical switch, link-local multicast packets are flooded across all physical ports in the VLAN.

Resolution

Ensure the physical switch is configured to restrict or prevent the flooding of link-local multicast frames to avoid duplicate packet delivery and potential network loops. Specific implementation steps vary depending on the physical switch vendor.