When the native VLAN on a switch is changed from the default of 1, Intel MT and PT dualport NICs drop all traffic with the new native VLAN tag. This results in CDP packets being dropped, due to which CDP appears not to function correctly.
This issue is resolved in ESX 4.0 Update 1. This fix adds checks for CDP packets with VLAN tags, sanity checks on incoming packets, and logging output for parsing errors.
Based on VMware KB 1006379