Keeping an up-to-date network inventory is essential for network efficiency, especially given current networks changing dynamics. The Cisco Discovery Protocol (CDP) is a network discovery tool, which assists network administrators and engineers in identifying neighboring Cisco devices, particularly those running lower-layer, transparent protocols.

CDP is a Layer 2 proprietary protocol that is default-enabled on all Cisco devices including routers and switches and runs on all LAN and WAN media that support Subnetwork Access Protocol (SNAP).

As part of its discovery protocol, CDP packets are sent to a multicast address every 60 seconds with each device enabled with at least one IP address that can receive SNMP messages. The information of each message will vary depending on the device and version of the operating system. The SNMP messages can include information such as hardware platform, IP addresses, hostname, duplex settings, and the active interfaces of CDP devices. The only requirement for running CDP is that the interface must support SNAP.

Cisco Discovery Protocol: Benefits and Features

CDP offers several benefits to service providers that need more visibility into their network, including:

  • Allowing the use of different network layer protocols and the real-time transport protocol to discover devices and how they are configured.
  • Enabling the discovery of the IP address of a misconfigured router or switch on the other side of a WAN link.
  • Assisting with troubleshooting Type-Length-Value (TLV) fields.

While Cisco Discovery Protocol should be enabled on all routers if you need to enable Cisco routers, use the router (config) # cdp run command. If you simply want to see the CDP configuration, use the router (config) # show cdp command.

CDP also works in an EXOS virtual machine environment that is running VMware. CDP not only allows you to determine which Cisco port is connected to a certain vSwitch, but to view properties of the Cisco switch such as the software version and device ID. While the CDP Version 1 prohibits native VLAN information to pass between Cisco switches, Version 2 can pass native VLAN information.

Cisco Discovery Protocol can be used as a diagnostic tool to help troubleshoot device and network-related issues. When used with the IEEE 801.1ab version of Link Layer Discovery Protocol (LLDP), a non-Cisco device can also advertise information to network devices.

How to Monitor with Cisco Discovery Protocol

While CDP is a discovery protocol tool that simplifies the process of keeping an up-to-date inventory of Cisco network devices, it only allows for Layer 2 discovery of physical connections. To gain a more granular view of your network topology, including Cisco devices, you will need to use network monitoring software that includes Layer 3 and Layer 2 discovery. Your network monitoring software should provide a log file and insights into the availability of all devices in the network, including device status, time to respond, and downtimes.

WhatsUp Gold features powerful Layer 2/3 discovery which can be initiated from an IP Range Scan or SNMP Smart Scan of a core router’s Bridge Table and generates a detailed map of the core, Windows, wireless and virtual infrastructures and more.


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