Configure Cisco CDP on ESX(i)
Before we start with configuring CDP on a ESX(i) hosts, let’s first take a look at what CDP realy is?
According to Cisco; The Cisco Discovery Protocol (CDP) is primarily used to obtain protocol addresses of neighboring devices and discover the platform of those devices. CDP can also be used to show information about the interfaces your router uses. CDP is media- and protocol-independent, and runs on all Cisco-manufactured equipment including routers, bridges, access servers, and switches.
Okay, now thats more clear (I hope so 
), lets take a look at how we can use CDP within ESX(i) because, hopefully as you all know by now, VMware ESX(i) is capable of using the Cisco Discovery Protocol (CDP).
By default the CDP configuration is in Listen only mode, this means that the ESX(i) host can only recieve CDP information like for example; on what switchport and switch it is connected, type of switch, VLAN, routing support etc. This is the same on a standard vSwitch as well as the distributed dvSwitch.
This is fine for an Administrator if he wants to know on what switchport an ESX(i) host is connected, but perhaps the network folkes also want to know the same information only from their perspective. Because the default setting is on “Listen”, the ESX(i) hosts won’t send any CDP information back to the switch, so if a network guy logs in his switch and execute the command: “show cdp neighbors” he won’t get any information that there is an ESXi hosts with a vSwitch or dvSwtich connected to a switch port.
So to enable CDP on “both” sides we need to configure CDP to Listen and Advertise at the same time so both parties have the information they need. But before we start configuring CDP, lets first take a look at what configuration options we have available.