Link Layer Discovery Protocol (LLDP) is a vendor-neutral data link layer protocol defined in IEEE standard 802.1AB. Just like Cisco’s CDP, LLDP is used by network devices to advertise their identity, capabilities, and neighbors on a local Ethernet network. However, since LLDP is an open standard, it has one big advantage over CDP – it can be used on non-Cisco devices.
Cisco LLDP neighbor discovery protocol is disabled by default on Cisco devices. To enable LLDP globally, use the lldp run command in the config mode:
To display information about the LLDP neighbors, run the show lldp neighbors command:
R1#show lldp neighbors Capability codes: (R) Router, (B) Bridge, (T) Telephone, (C) DOCSIS Cable Device (W) WLAN Access Point, (P) Repeater, (S) Station, (O) Other Device ID Local Intf Hold-time Capability Port ID R2 Gig0/0 120 R Gig0/0 Total entries displayed: 1
As you can see from the output above, R1 has a single neighbor. Here is the description of the fields in the output:
- Device ID – the neighbor’s host name
- Local Intf – the local device interface on which the neighbor is connected to
- Hold-time – the LLDP timer in which the receiving device should hold the information sent by this device before discarding it.
- Capability – the type of the device. R means router, T means telephone, etc.
- Port ID – neighboring device’s interface
To get more detailed information about LLDP neighbors, run the show lldp neighbors detail command:
R1#show lldp neighbors detail ------------------------------------------------ Chassis id: 0002.4A13.6C01 Port id: Gig0/0 Port Description: GigabitEthernet0/0 System Name: R2 System Description: Cisco IOS Software, C1900 Software (C1900-UNIVERSALK9-M), Version 15.1(4)M4, RELEASE SOFTWARE (fc2) Technical Support: http://www.cisco.com/techsupport Copyright (c) 1986-2012 by Cisco Systems, Inc. Compiled Thurs 5-Jan-12 15:41 by pt_team Time remaining: 90 seconds System Capabilities: R Enabled Capabilities: R Management Addresses - not advertised Auto Negotiation - supported, enabled Physical media capabilities: 1000baseT(FD) 100baseT(FD) Media Attachment Unit type: 10 Vlan ID: 1 Total entries displayed: 1
To get information about a specific neighbor, run the show lldp entry DEVICE_ID command.
To display global LLDP information, run the show lldp command:
R2#show lldp Global LLDP Information: Status: ACTIVE LLDP advertisements are sent every 30 seconds LLDP hold time advertised is 120 seconds LLDP interface reinitialisation delay is 2 seconds
This command displays information about whether LLDP is active on the device, the frequency of LLDP updates and transmissions, the hold time for the packets being sent, and the delay time for LLDP to initialize on an interface.
You can also configure whether you would globally disable LLDP or would like your device to send LLDP packets and receive them on a particular interface using the no lldp transmit and no lldp receive interface mode commands. For example, to only receive LLDP packets on the Gi0/0 interface, I would use the following LLDP configuration to disable LLDP:
R1(config-if)#no lldp transmit
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