Interfaces and Hardware Component Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

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Interfaces and Hardware Component Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

Link Layer Discovery Protocol

Updated: July 31, 2026

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Introduces the IEEE 802.1AB Link Layer Discovery Protocol (LLDP), which the router supports in addition to Cisco Discovery Protocol (CDP) to enable interoperability with non-Cisco devices.

The Cisco Discovery Protocol (CDP) is a device discovery protocol that

  • runs over Layer 2 on all Cisco-manufactured devices, such as routers, bridges, access servers, and switches

  • allows network management applications to automatically discover and learn about Cisco devices, and

  • supports network visibility.

To support non-Cisco devices and to allow for interoperability between other devices, the router also supports the IEEE 802.1AB Link Layer Discovery Protocol (LLDP). LLDP is a neighbor discovery protocol that

  • supports interoperability between non-Cisco devices

  • advertises device information to other network devices, and

  • operates over the Data Link Layer.

Feature history

Feature name

Release information

Feature description

LLDP snooping

 Release 25.4.1

Introduced in this release on: Fixed Systems (8010 [ASIC: A100])(select variants only*)

*This feature is now supported on:

  • 8011-32Y8L2H2FH

  • 8011-12G12X4Y-A/D

LLDP snooping

 Release 25.1.1

Introduced in this release on: Fixed Systems (8700 [ASIC: K100], 8010 [ASIC: A100])(select variants only*)

*This feature is supported on:

  • 8712-MOD-M

  • 8011-4G24Y4H-I

LLDP snooping

 Release 24.4.1

Introduced in this release on: Fixed Systems (8200 [ASIC: P100], 8700 [ASIC: P100])(select variants only*); Modular Systems (8800 [LC ASIC: P100])(select variants only*)

*This feature is now supported on:

  • 8212-48FH-M

  • 8711-32FH-M

  • 88-LC1-12TH24FH-E

  • 88-LC1-36EH

  • 88-LC1-52Y8H-EM

LLDP snooping

 Release 7.3.3

With this release, you can further leverage the Link Layer Discovery Protocol (LLDP) information for directly attached devices or equipment in an L2 (Layer 2) network via LLDP snoop. In order to utilize the LLDP snoop functionality, the neighbouring devices must exchange the LLDP packets with the L2 network.

With the help of the LLDP snoop functionality, you can identify the cabling and modeling failures and isolate faults.

To enable LLDP snoop, enable the LLDP command on an interface while the outgoing (TX) traffic is disabled.

TLV attributes and mandatory TLVs

LLDP uses a set of attributes known as Type-Length-Value (TLV) to exchange information between neighbor devices, such as configuration information, device capabilities, and device identity. In addition to the mandatory TLVs, such as Chassis ID, Port ID, End of LLDPDU, and Time-to-Live, the router supports the following basic management TLVs:

  • Port Description

  • System Name

  • System Description

  • System Capabilities

  • Management Address

These optional TLVs are automatically sent when LLDP is active, but you can disable them as needed using the lldp tlv-select Optional TLVdisable command.

MACsec encrypts LLDP packets by default. You can enable exceptions in the MACsec policy using the allow lldp-in-clear command to retain the LLDP packets unencrypted with MACsec. For more information, see MACsec Policy Exceptions for Link Layer Discovery Protocol Packets section in the Configuring MACsec chapter of the System Security Configuration Guide for Cisco 8000 Series Routers.

Note

For LLDP to work on any bundle member, enable LLDP on the bundle main interface either globally or on the interface itself. You can then choose to disable LLDP transmission on bundle main interface by using the lldp transmit disable command.

You can also control LLDP transmit or receive on each bundle member interface as desired.

LLDP frame format

LLDP frames use the IEEE 802.3 format, which consists of these fields:

  • Destination address (6 bytes)—Uses a multicast address of 01-80-C2-00-00-0E.

  • Source address (6 bytes)—MAC address of the sending device or port.

  • LLDP Ethertype (2 bytes)—Uses 88-CC.

  • LLDP PDU (1500 bytes)—LLDP payload consisting of TLVs.

  • FCS (4 bytes)—Cyclic Redundancy Check (CRC) for error checking.

LLDP TLV format

LLDP TLVs carry the information about neighboring devices within the LLDP PDU using this basic format:

  • TLV Header (16 bits), which includes these fields:

    • TLV Type (7 bits)

    • TLV Information String Length (9 bits)

  • TLV Information String (0 to 511 bytes)

LLDP operation

The Link Layer Discovery Protocol (LLDP) is a one-way protocol that

  • sends periodic advertisements in LLDP frames to receiving devices

  • identifies devices using a combination of Chassis ID and Port ID to create an MSAP (MAC Service Access Point), and

  • stores neighbor information in a remote cache until the TTL expires.

Operational characteristics

LLDP supports these additional operational characteristics:

  • LLDP can operate independently in transmit or receive modes. On global lldp enablement, the default mode is to operate in both transmit and receive modes.

  • LLDP operates as a slow protocol with transmission speeds not greater than one frame per five seconds.

  • LLDP packets are sent during these scenarios:

    • The packet update frequency specified by the lldp timer command is reached. The default is 30 seconds.

    • When a change in the values of the managed objects occurs from the local system’s LLDP MIB.

    • When LLDP is activated on an interface (3 frames are sent upon activation similar to CDP).

  • When an LLDP frame is received, the LLDP remote services and PTOPO MIBs are updated with the information in the TLVs.

  • LLDP supports the following actions on these TLV characteristics:

    • Interprets a neighbor TTL value of 0 as a request to automatically purge the information of the transmitting device. These shutdown LLDPDUs are typically sent prior to a port becoming inoperable.

    • An LLDP frame with a malformed mandatory TLV is dropped.

    • A TLV with an invalid value is ignored.

    • A copy of an unknown organizationally-specific TLV is maintained if the TTL is non-zero, for later access through network management.

Supported LLDP functions

The router supports these LLDP functions:

  • The router advertises both IPv4 and IPv6 addresses if they are available, and the system gives preference to the address that is configured on the transmitting interface.

    If the transmitting interface does not have a configured address, then the system populates the TLV with an address from another interface. The advertised LLDP IP address is implemented according to the following priority order of IP addresses for interfaces on the router:

    • Locally configured address on the transmitting interface

    • MgmtEth0/RP0/CPU0/0

    • MgmtEth0/RP0/CPU0/1

    • MgmtEth0/RP1/CPU0/0

    • MgmtEth0/RP1/CPU0/1

    • Loopback interfaces

      There are some differences between IPv4 and IPv6 address management in LLDP:

  • The router populates the TLV with an address from another interface if the transmitting interface does not have a configured address.

  • For IPv4, as long as the IPv4 address is configured on an interface, it can be used as an LLDP management address.

  • For IPv6, after the IPv6 address is configured on an interface, the interface status must be Up and pass the DAD (Duplicate Address Detection) process before it is can be used as an LLDP management address.

  • LLDP is supported for the nearest physically attached, non-tunneled neighbors.

  • LLDP is supported for Ethernet interfaces, L3 subinterfaces, bundle interfaces, and L3 bundle subinterfaces.

  • LLDP snoop is supported on L2 interfaces, when the incoming (Rx) traffic is enabled and outgoing (Tx) traffic is disabled.

Unsupported LLDP functions

These are the unsupported LLDP functions on the router:

  • LLDP-MED organizationally unique extension—however, interoperability still exists between other devices that do support this extension.

  • Tunneled neighbors, or neighbors more than one hop away.

  • LLDP TLVs cannot be disabled on a per-interface basis; However, certain optional TLVs can be disabled globally.

  • LLDP SNMP trap lldpRemTablesChange.

  • LLDP is not supported on the FP-X line cards.

LLDP default configuration

LLDP default values

This table shows the values of the LLDP default configuration on the router. To change the default settings, use the LLDP global configuration and LLDP interface configuration commands.

LLDP function

Default

LLDP global state

Disabled

LLDP holdtime (before discarding)

120 seconds

LLDP timer (packet update frequency)

30 seconds

LLDP reinitialization delay

2 seconds

LLDP TLV selection

All TLVs are enabled for sending and receiving.

LLDP interface state

Enabled for both transmit and receive operation when LLDP is globally enabled.

User-defined LLDP TLV values

It is possible to override the system default values for some of the mandatory LLDP Type-Length-Values (TLVs) that are advertised by routers to their directly connected neighboring devices. The LLDP TLV values are network management settings that

  • allow routers to override default system values

  • enable the assignment of user-defined identification names, and

  • facilitate the transmission of custom identity data to neighboring devices.

While advertising their identity and capabilities, routers can assign user-defined meaningful names instead of autogenerated values. Using these CLIs, you can specify the user-defined values:

  • Router(config)#lldp system-name system-name

  • Router(config)#lldp system-description system-description

  • Router(config)#lldp chassis-id-type chassis-type

  • Router(config)#lldp chassis-id local-chassis-id

    Note

    The chassis-id value is configurable only when the chassis-id-type is set as Local . If there is a mismatch, you encounter a configuration failed error message.

The configured values, such as the system name, system description, chassis-id, chassis-type become part of the TLV in the LLDP packets that are sent to its neighbors. Values are transmitted only to LLDP enabled interfaces to which the router is connected.

chassis-id-type values

The chassis-id-types are objects that are part of the management information base (MIB). Depending on the selected chassis-id-types, values are assigned to these objects, and they are advertised by the router to its neighboring devices. You can assign any of the values for the chassis-id-type as described in this table.

chassis-id-type

Description

chassis-component

Chassis identifier based on the value of entPhysicalAlias object that is defined in IETF RFC 2737.

interface-alias

Chassis identifier based on the value of ifAlias object as defined in IETF RFC 2863.

interface-name

Chassis identifier based on the name of the interface.

local

Chassis identifier based on a locally defined value.

mac-address

Chassis identifier based on the value of a unicast source address.

network-address

Chassis identifier based on a network address that is associated with a particular chassis.

port-component

Chassis identifier based on the value of entPhysicalAlias object defined in IETF RFC 2737 for a port or backplane component.
Tip

You can programmatically modify default values of LLDP TLVs by using the openconfig-lldp OpenConfig data model. To get started with using data models, see the Programmability Configuration Guide for Cisco 8000 Series Routers.

Example: Configure user-defined LLDP TLVs

This example shows the configuration for the LLDP TLVs that will be advertised by routers to their directly connected neighboring devices.

Router(config)#lldp system-name cisco-xr
Router(config)#lldp system-description cisco-xr-edge-device
Router(config)#lldp chassis-id-type local
Router(config)#lldp chassis-id ce-device9

The following running configuration shows the resulting show lldp output on the router:

Router#show lldp
Tue Sep 13 16:03:44.550 +0530
Global LLDP information:
Status: ACTIVE
LLDP Chassis ID: ce-device9 
LLDP Chassis ID Subtype: Locally Assigned Chassis Subtype 
LLDP System Name: cisco-xr 
LLDP advertisements are sent every 30 seconds
LLDP hold time advertised is 120 seconds
LLDP interface reinitialisation delay is 2 seconds