Day-27:LLDP(Link Layer Discovery Protocol )

By | June 11, 2022
  • LLDP is an Open standard protocol, equivalent to CDP
  • Defined in IEEE 802.1ab
  • Media Endpoint Discovery (MED) is an LLDP enhancement for Voice over IP (VoIP) applications.
  • Limited to only 802.1 media types (i.e. Ethernet…but not WAN interfaces)
  • CDP and LLDP can be operational on the same interface.

LLDP uses attributes that contain type, length, and value descriptions(TLV ). These are called TLVs (Type, Length, Value). Devices that support LLDP use TLVs to send and receive information to their directly connected neighbors. Here’s an example of some basic TLVs:

  • Port description TLV
  • System name TLV
  • System description TLV
  • System capabilities TLV
  • Management Address TLV

How to globally disable LLDP.

Switch# configure terminal
Switch(config)# no lldp run
Switch(config)# end

How to globally enable LLDP.

Switch# configure terminal
Switch(config)# lldp run
Switch(config)# end

let’s verify the LLDP details.

SW1#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
SW2                 Fa0/24         120        B               Fa0/24

Total entries displayed: 1

if you want to see more information on lldp then you can see the neighbor details.

SW1#show lldp neighbors detail


Chassis id: 0011.bb0b.361a
Port id: Fa0/24
Port Description: FastEthernet0/24
System Name: SW2.cisco.com

System Description:
Cisco IOS Software, C3560 Software (C3560-ADVIPSERVICESK9-M), Version 12.2(46)SE, RELEASE SOFTWARE (fc2)
Copyright (c) 1986-2008 by Cisco Systems, Inc.
Compiled Thu 21-Aug-08 15:26 by nachen

Time remaining: 106 seconds
System Capabilities: B,R
Enabled Capabilities: B
Management Addresses - not advertised
Auto Negotiation - supported, enabled
Physical media capabilities:
    100base-TX(FD)
    100base-TX(HD)
    10base-T(FD)
    10base-T(HD)
Media Attachment Unit type: 16
---------------------------------------------


Total entries displayed: 1

Monitoring and Maintaining LLDP and LLDP-MED

CommandDescription
clear lldp countersResets the traffic and error counters to zero.
clear lldp tableDeletes the LLDP table of information about neighbors.
show lldpDisplays global information, such as frequency of transmissions, the holdtime for packets being sent, and the delay time for LLDP to initialize on an interface.
show lldp entry entry-nameDisplays information about a specific neighbor.You can enter an asterisk (*) to display all neighbors, or you can enter the name of the neighbor about which you want information.
show lldp errorsDisplays LLDP computational errors and overflows.
show lldp interface [interface-id]Displays information about interfaces where LLDP is enabled.You can limit the display to the interface about which you want information.
show lldp neighbors [interface-id] [detail]Displays information about neighbors, including device type, interface type and number, holdtime settings, capabilities, and port ID.You can limit the display to neighbors of a specific interface or expand the display to provide more detailed information.
show lldp trafficDisplays LLDP counters, including the number of packets sent and received, number of packets discarded, and number of unrecognized TLVs.

Disabling and Enabling LLDP on an Interface

You can configure the interface to selectively not to send and receive LLDP packets with the no lldp transmit and no lldp receive commands.

This example shows how to enable LLDP on an interface.

Switch# configure terminal
Switch(config)# interface GigabitEthernet 1/1
Switch(config-if)# lldp transmit
Switch(config-if)# lldp receive
Switch(config-if)# end
This example shows how to Disable LLDP on an interface.

Switch# configure terminal
Switch(config)# interface GigabitEthernet 1/1
Switch(config-if)#no lldp transmit
Switch(config-if)# no lldp receive
Switch(config-if)# end

Specific TLV enabling on an interface when it has been disabled.

Switch# configure terminal
Switch(config)# interface GigabitEthernet1/0/1
Switch(config-if)# lldp med-tlv-select inventory management
Switch(config-if)# end

LLDP Timers.

  • Hold timer refers to the time or duration that an LLDP device maintains the neighbor information before aging it. The default is 120 seconds. If the timer expires and no LLPD packet was received, the neighbor information will be discarded.
  • The frequency timer pertains to the interval at which the network devices send LLDP updates to neighboring devices. The default is 30 seconds.
  • A Reinit timer is the delay time for LLDP to initialize on any interface. The default is 2 seconds.

The reinitiate timer specifies the number of seconds the device delays before sending advertisements to neighbors after a change is made in a TLV (type, length, or value) element in the Link Layer Discovery Protocol (LLDP) or in the state of the local system, such as a change in hostname or management address. You can set this value to reduce the delay in notifying neighbors of a change in the local system.

This is basically a Delay after a change to the local TLVs or system state before LLDP advertisements are sent.

Switch# configure terminal
Switch(config)# lldp holdtime 120
Switch(config)# lldp reinit 2
Switch(config)# lldp timer 30
Switch(config)# end

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