It's recommended that as a network admin, you should consider that monitoring of "ESXi hardware usage and network transmit" as one of your virtual infrastructure management phases. Regardless of using monitoring tools or not, SNMP Traffic that is generated from your host, maybe face with an error. After reviewing your "community string" (SNMP v1/v2) or "credential" (SNMP v3) and checking network connection, if still there is a problem, you can execute an useful command for SNMP traffic inspection.
After logging to ESXi Host directly (DCUI) or by SSH connection (e.g Putty) , run this command to resolve the problem:
tcpdump-uw -vvv -i vmk0 -T snmp udp and port 162
Therefore you will see each SNMP UDP packets that are transferred on port 162. Also note this repeated "-vvv" syntax, which means you want to see more information of your command's result. Literally you can put only "-v" or "-vv" on your command.
Source of content inside my personal blog: Undercity of Virtualization: Analyze SNMP Traffic inside the ESXi