Multicast packets passing through an ENS enabled switch are truncated by the flow cache processing.

Products

• VMware NSX Networking
• VMware NSX-T Data Center
• VMware NSX

Issue/Introduction

Multicast packets are truncated
Multicast communications such as used in VRRP or OSPF are not properly functioning
VDS or N-VDS with ENS enabled
Packet capture at vmnic shows VRRP payload is truncated

Environment

NSX 3.2.x
ESXi 6.7

Cause

This issue occurs due to improper handling of flowcache for multicast packets in ENS environment on ESXi 6.7.
This issue does not occur in ESXi 7.0 and later because the processing is different.
This issue does not occur in NSX-T 3.1 and earlier because flowcache for multicast packets in ENS is not supported.

Resolution

Disable multicast flowcache on ESXi host with the following steps.

1. Login to ESXi host as root.
2. Execute the following CLI command to disable multicast flowcache temporarily.
   
   # net-dvs -u 'com.vmware.net.portset.fc.mcast.enabled' -p hostPropList <dvs-name>
   Note: This change will not persist across host reboots.
   
   
3. To persistently disable, edit the "/etc/vmware/nsx/nsx-cfgagent.xml" as below.
   
   

   <Before>
===================================================
<flowCache>
<enabled>true</enabled>
<mcastEnabled>true</mcastEnabled> <--- 
</flowCache>
===================================================

   <After>
===================================================
<flowCache>
<enabled>true</enabled>
<mcastEnabled>false</mcastEnabled> <---
</flowCache>
===================================================