CDP & LLDP

Day 36

Layer 2 discovery protocols (CDP and LLDP) share information with and discover information about neighbouring devices. The shared information includes hostname, IP address, device type, etc.

CDP

CDP (Cisco Discovery Protocol) is a Cisco proprietary protocol which is enabled on all Cisco devices by default. The default version is CDPv2. CDP messages are periodically sent to multicast MAC address 0100.0ccc.cccc. When a device receives a CDP message, it processes and discards the message without forwarding it to other devices. By default, CDP messages are sent every 60 seconds. The default hold time is 180 seconds.

Verification

To see CDP configurations, enter the show cdp command, for CDP traffic - show cdp traffic, and for CDP information about each interface - show cdp interface. To see the CDP neighbours, enter the command show cdp neighbors or show cdp neighbors detail, for the specified neighbour - show cdp entry followed by the hostname.

Configuration

  • Enable/disable CDP globally - cdp run/no cdp run.

  • Enable/disable CDP on a specific interface - cdp enable/no cdp enable.

  • Configure the CDP timer - cdp timer followed by the value in seconds.

  • Configure the CDP hold time - cdp holdtime followed by the value in seconds.

  • Enable/disable CDPv2 - cdp advertise-v2/no cdp advertise-v2.

LLDP

LLDP (Link Layer Discovery Protocol) is an industry-standard protocol (IEEE 802.1ab). It is usually disabled on Cisco devices by default. A device can run CDP and LLDP at the same time. LLDP messages are multicast to MAC address 0180.c200.000e. By default, LLDP messages are sent every 30 seconds and holdtime is 120 seconds. It also has an additional timer - reinitialization delay. When LLDP is enabled, this timer delays the actual initialization of LLDP. It is 2 seconds by default.

Verification

The commands and outputs are very similar to CDP.

  • For basic information - show lldp.

  • For LLDP stats - show lldp traffic.

  • For LLDP-enabled interfaces - show lldp interface.

  • For LLDP neighbours - show lldp neighbors or show lldp neighbors detail, for the specified neighbour - show lldp entry followed by the hostname.

Configuration

  • Enable LLDP globally - lldp run.

  • Enable LLDP to transmit on a specific interface - lldp transmit.

  • Enable LLDP to receive on a specific interface - lldp receive.

  • Configure the LLDP timer - lldp timer followed by the value in seconds.

  • Configure the LLDP hold time - lldp holdtime followed by the value in seconds.

  • Configure the LLDP reinit timer - lldp reinit followed by the value in seconds.

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