Link Layer Discovery Protocol (LLDP) is a vendor-neutral 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.
LLDP is disabled by default on Cisco devices. To enable it, 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 time 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 detail 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 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 transmissions, the holdtime for packets being sent, and the delay time for LLDP to initialize on an interface.
You can also configure whether you would like your device to send or receive LLDP packets 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 command to disable the sending of LLDP packets:
R1(config-if)#no lldp transmit