VMware ESX v4.x includes an SNMP agent that is embedded in hostd service. This VMware SNMP agent supports VMware Management Information Base (MIB) files and extends SNMP functionality.
Before version 4, VMware ESX used Net-SNMP as SNMP agent. Now it has its own agent which can be used with or without Net-SNMP. Usually, Net-SNMP is required by monitoring software. In this case it can be set up to pass any incoming requests, related to VMware, to VMware SNMP agent to get access to VMware MIBs.
This article describes how to configure both agents. If you do not need to use VMware SNMP agent, configure Net‐SNMP as you would do on a typical Linux host. If you do not need to use Net-SNMP agent, configure VMware SNMP agent to use UDP port 161 instead of 171, shown in this document
Some information can be also found here: http://www.vmware.com/pdf/vsp_4_snmp_config.pdf