The Link Layer Discovery Protocol (LLDP) is a vendor-neutral link layer protocol in the Internet Protocol Suite used by network devices for advertising their identity, capabilities, and neighbors at Layer 2 on an IEEE 802 local area network, principally wired Ethernet. The protocol’s multi-vendor support enables network discovery of devices and management tools such as Simple Network Management Protocol ( SNMP) in a network made up of devices from different manufacturers.
- Type code is used to specify what kind of information is included in this TLV. For example, for LLDP, a code of 2 means the TLV has information about the Port ID.
- Length indicates the length of the field containing the value, in bytes.
- Value—this field contains the value of the TLV. If the Type code is 2, then you can expect to see the Port ID in this field.
TLVs for LLDP exist within the LLDP frame structure. An example of an LLDP frame is shown below:
LLDP enables defining optional TLV units by using organization unique identifiers (OUI)s or organizationally-specific TLVs. An OUI identifies the category for a TLV unit depending on whether the OUI follows the IEEE 802.1 or IEEE 802.3 standard. These TLVs are IEEE 802.1 or IEEE 802.3 organizationally-specic.
The optional TLVs are basic and organizationally -specific. The following table lists all optional TLVs with their names, units, and description.
| TLV Group | TLV Name | TLV units | Description |
|---|---|---|---|
| Basic management | basic-tlv | port-descr, sys-name, sys-descr, sys-capab, and mgmt-addr | Specifies the system name, port description, system capability, system description, and management address to be advertised. |
| Organizationally - specific 802.1 OUI | dot1-tlv | vlan-name, port-vlan-id, protocol-vlan-id, vid-usage-digest, link-aggregation, mgmt-vid | Specifies advertising of VLAN ID, protocol VLAN name, port VLAN ID, link aggregation, VID usage digest, and management VID. |
| Organizationally - specific 802.3 OUI | dot3-tlv | max-framesize, macphy-config, and link-aggregation | Specifies advertising of maximum frame size, MAC or PHY TLV, and link aggregation. |
If the LLDP PDU includes optional TLVs, they will be inserted between the Time-To-Live TLV and End-of- LLDPDU TLV.
The transmission status of the optional TLVs is disabled by default and they have to be enabled. See sections “Enabling LLDP Basic Optional TLVs” and “Enabling IEEE 802.1 Organizationally Specific TLVs”.
The next image shows an Wireshark capture of LLDP frame which illustrates all available TLVs.
Custom TLVs can also be created by using type number 127 where each vendor can specify their own information within the value field.
The device’s LLDP Admin Status can be configured or it can be configured if a device is to send or receive LLDP packets on a particular interface using the “no lldp transmit” and “no lldp receive” Interface Mode commands (refer to section Configuring LLDP Admin Status).
LLDP-MED
LLDP does not contain the capability of negotiating additional information such as PoE management and VLAN assignments. This capability was added as an enhancement known as Media Endpoint Discovery or MED, resulting in the enhanced protocol LLDP- MED.
The MED enhancement has been standardized by the Telecommunications Industry Association in standard number ANSI/TIA-1057.
This chapter describes the purpose and scope of the document, acronyms, and general configurations used in this document.