What is Cisco Discovery Protocol and How does it work?

What is Cisco Discovery Protocol?

    The Cisco Discovery Protocol (CDP) is a proprietary Data Link Layer protocol developed by Cisco Systems and is used in almost all the Cisco Devices. It is used to share the information about the other directly connected Cisco devices, such as the operating system version and IP address. The CDP Protocol is Mainly used for troubleshooting purposes in networks. The CDP is a proprietary Data link Layer Protocol developed by Cisco, where HP, IBM uses LLDP (Link Layer Discovery Protocol)

       How does the CDP Work?
        
      Since CDP runs on Layer 2 no IP address mechanisms are used

      • Cisco devices send CDP announcements to the multicast destination address (MAC)01-00-0c-cc-cc-cc, out each connected network interface.
      • Uses MAC Multicast addresses to send and retrieve the packets
      • A CDP enabled device sends CDP Notifications every 60 seconds
      • Received packets are stored internally
      • The CDP table information is refreshed each time an announcement is received, and the holdtime for that entry is reinitialized.
      • The holdtime specifies the lifetime of an entry in the table – if no announcements are received from a device for a period in excess of the holdtime, the device information is discarded (default 180 seconds.
      • The information contained in CDP announcements varies by the type of device and the version of the operating system running on it. This information may include the operating system version, hostname, every address (i.e. IP address) from all protocol(s) configured on the port where CDP frame is sent.
      • Recommended to be disabled in an secure environment (i.e. DMZ)
      Using CDP with VMware ESX/ESXi?

      • When Cisco Discovery Protocol is enabled on ESX/ESXi, it will include information on which vmnics are connected to which switch ports.
      • It is impossible to identify the switch port for the ESX NICs without “follow the cable”, this is where CDP was implemented and was available since the release of ESX 3.5
      • Helpful for troubleshooting the Network related issues and helps you locating the physical switch to which the ESX Server is connected to.
       How to manage CDP on ESX?
      The CDP works in three modes:
      • listen (default)
      • advertise
      • both
      Listen mode – The ESXi/ESX host detects and displays information about the associated Cisco switch port, but information about the vSwitch is not available to the Cisco switch administrator.
      Advertise mode – The ESXi/ESX host makes information about the vSwitch available to the Cisco switch administrator, but does not detect and displays information about the Cisco switch.
      Both mode – The ESXi/ESX host detects and displays information about the associated Cisco switch and makes information about the vSwitch available to the Cisco switch administrator.
      How to check the CDP information?
      To view CDP information using the vSphere/VI Client:

      1. Connect to the vCenter Server/Virtual Center or ESX host using the vSphere/VI Client.
      2. Select the ESX host in the Inventory.
      3. Click the Configuration tab.
      4. Click Networking.
      5. Click the Info icon to the right of the vSwitch, under Properties. For example:

      6. A tool tip opens with the Cisco Discovery Protocol information for the selected physical network interface. For example:
       Here are the steps to configure CDP on a vNetwork Distributed Switches (vDS):
      1. Connect to vCenter Server using the vSphere Client.
      2. In the vCenter Server home page, click Networking.
      3. Right-click the vDS and click Edit Settings.
      4. Select Advanced under Properties.
      5. Using the checkbox and the dropdown, change the CDP settings.

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