I received a question last week from a customer who had implemented a stretched VSAN cluster. The Health Check after the implementation indicated that there was an “issue” with the MTU configuration. The customer had explained that he had configured an MTU of 9000 between the two data sites and an MTU of (default) 1500 between data sites and the witness.
The question of course was, why the Health Check indicated there was an issue. The problem here is that witness traffic and data in todays version of Virtual SAN use the same VMkernel interface. If the VSAN VMkernel interface on the the “data” site is configured for 9000 and one the “witness” site is configured for 1500 then there is a mismatch which causes fragmentation etc. This is what the health check calls out. VSAN (and the health check as such) expects an “end-to-end” consistently configured MTU, even in a stretched environment.