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ThinkMo EDU Share – network 1. CDP Cisco discovery protocol

ThinkMo No Comments 10/13/2022

ThinkMo EDU Share – network 1. CDP Cisco discovery protocol

Overview of CDP protocol

Cisco Discovery Protocol (CDP) is a Cisco-owned protocol and can only be used on Cisco devices.

The purpose of this protocol is to help network management and network engineers understand the network topology and the status of network equipment, and to provide a simple method to locate the fault point for troubleshooting network equipment.

CDP protocol works on the data link layer of OSI reference model, and it has nothing to do with the protocol of the upper layer and the physical connection medium, that is, no matter what protocol stack is adopted in the upper layer, the status of the network equipment can be known through CDP protocol. As long as the physical layer connection conforms to SNAP standard, CDP protocol can work on these connection media, including Ethernet, Frame Relay, ATM, etc.

CDP version 2(CDPv2) is the latest version of this protocol.

Using CDP protocol, you can know a lot of useful information on all devices adjacent to the device that the user is configuring, including device identification, device address, device platform, device function, device interface, holding timer, IOS version and CDP version.

Devices configured with CDP periodically send information (i.e., announcements) to multiple routers. Every other cycle, the device advertises at least one address, at which it can receive SNMP messages. Of course, this announcement also includes the information of survival time and retention time. In addition, each device will listen to CDP neighbor information periodically sent by other devices to learn from neighboring devices.

By default, CDP sends information every 60 seconds. The hold timer is reset every time a CDP message is received. The timer can detect when the neighbor device crashes or does not respond because of the table. When the hold timer expires, the device is considered to be dead, and it is deleted from CDP’s neighbor list. By default, the hold timer is set to 180 seconds or three times the period of sending information.

Configure CDP protocol features

  1. To specify the period for IOS to send CDP updates, in the global configuration mode, use the cdp timer command. To return the default value, use the no form of this command.

Cdp timer [seconds] //seconds: the period for sending CDP updates, which is 60 seconds by default.

no cdp timer

  1. To specify the time for the receiving device to keep the CDP packet before dropping it, in the global configuration mode, use the cdp holdtime command. To return to the default configuration, use the no form of this command.

Hold time [seconds]//seconds: the holding time of CDP packets, which is 180 seconds by default.

no cdp holdtime

  1. On Cisco devices, the broadcast of CDPv2 advertisement is enabled by default. To enable the CDPv2 advertisement function on the device, in the global configuration mode, use the cdp advertise-v2 command, and to disable the CDPv2 advertisement function, use the no form of this command.

cdp advertise – v2

no cdp advertise – v2

Or disable CDP protocol.

  1. On Cisco devices, CDP is enabled by default. To turn on CDP, in global configuration mode, use the cdp run command. To disable CDP, use the no form of this command.

cdp run

no cdp run

  1. On all supported interfaces (except Frame Relay multipoint subinterfaces), CDP is enabled by default and can send and receive CDP information. To enable CDP on an interface, in interface configuration mode, use the cdp enable command. To disable CDP on an interface, use the no form of this command.

cdp enable

no cdp enable

There are four reasons for disabling CDP: saving network bandwidth, saving CPU resources, and connecting non-Cisco devices to ensure security.

Monitor and maintain CDP protocol

  1. To display global CDP information, including timer and hold time information, use the command in privileged mode:

show cdp

  1. To display information about the specified neighbor devices discovered through CDP, use in privileged mode:

show cdp entry { * | device – name[*]}[version][protocol]

  1. To display information about enabling CDP interface, in privileged mode, use the command:

show cdp interface [type number]

  1. To display the details of neighbors discovered through CDP, in privileged mode, use the command:

show cdp neighbors [type number] [detail]

  1. To display the information of CDP traffic, in privileged mode, use the command:

show cdp traffic

  1. To reset CDP traffic counter to 0, in privileged mode, use the command:

clear cdp counters

  1. To clear the table containing CDP information about neighbors, in privileged mode, use the command:

clear cdp table

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