LLDP frames use the IEEE 802.3 format, which consists of the following 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 TLVs carry the information about neighboring devices within the LLDP PDU using the following basic format:

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

  • TLV Type (7 bits)

  • TLV Information String Length (9 bits)

• TLV Information String (0 to 511 bytes)

LLDP is a one-way protocol. The basic operation of LLDP consists of a device enabled for transmit of LLDP information sending periodic advertisements of information in LLDP frames to a receiving device.

Devices are identified using a combination of the Chassis ID and Port ID TLVs to create an MSAP (MAC Service Access Point). The receiving device saves the information about a neighbor for a certain amount time specified in the TTL TLV, before aging and removing the information.

LLDP supports the following additional operational characteristics:

• LLDP can operate independently in transmit or receive modes.

• LLDP operates as a slow protocol using only untagged frames, with transmission speeds of less than 5 frames per second.

• LLDP packets are sent when the following occurs:

  • 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 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.

• IPv4 and IPv6 management addresses—In general, both IPv4 and IPv6 addresses will be advertised if they are available, and preference is given to the address that is configured on the transmitting interface.

  If the transmitting interface does not have a configured address, then the TLV will be populated 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

  • MgmtEth0/RP0/CPU0/0

  • MgmtEth0/RP0/CPU0/1

  • Loopback interfaces

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

• 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 and tunneled neighbors.

• Port ID TLVs are supported for Ethernet interfaces, subinterfaces, bundle interfaces, and bundle subinterfaces.

These LLDP functions are not supported:

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

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

• LLDP SNMP trap lldpRemTablesChange.