This will be a really quick heads-up for those upgrading vSphere 6 to vSphere 7.
I've been informed by one colleague, that his customer had an network outage when he upgraded VMware Distributed Switch (aka VDS) from version 6.6.0 (vSphere 6.7 U3) to 7.0.2 (vSphere 7.0 U2).
That was a surprise, as we were not aware about any VDS upgrade issues in the past.
The network outage was observed on Microsoft Network Load Balancers (aka NLB) which was a pretty good hint for Root Cause Analysis.
After the further analysis, the root cause was the change of VMware DVS default advanced setting "Multicast filtering mode".
In vSphere 6.7, the default "Multicast filtering mode" is basic.
In vSphere 7.0, the default "Multicast filtering mode" is IGMP/MLD Snooping.
For those who know how IGMP Snooping works, it is not a big surprise why it might be problem for Microsoft Network Load Balancer.
Hope this will help broader VMware community.