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Chapter 67: LLDP and LLDP-MED
1020
Overview
Link Layer Discovery Protocol (LLDP) and Link Layer Discovery Protocol
for Media Endpoint Devices (LLDP-MED) allow Ethernet network devices,
such as switches and routers, to receive and transmit device-related
information to directly connected devices on the network that are also
using the protocols, and to store the information that is learned about other
devices. The data sent and received by LLDP and LLDP-MED are useful
for many reasons. The switch can discover other devices directly
connected to it. Neighboring devices can use LLDP to advertise some
parts of their Layer 2 configuration to each other, enabling some kinds of
misconfiguration to be more easily detected and corrected.
LLDP is a “one-hop” protocol; LLDP information can only be sent to and
received by devices that are directly connected to each other, or
connected via a hub or repeater. Devices that are directly connected to
each other are called neighbors. Advertised information is not forwarded
on to other devices on the network. LLDP is a one-way protocol. That is,
the information transmitted in LLDP advertisements flows in one direction
only, from one device to its neighbors, and the communication ends there.
Transmitted advertisements do not solicit responses, and received
advertisements do not solicit acknowledgements. LLDP cannot solicit any
information from other devices. LLDP operates over physical ports only.
For example, it can be configured on switch ports that belong to static port
trunks or LACP trunks, but not on the trunks themselves. In addition, LLDP
can be configured on switch ports that belong to VLANs, but not on the
VLANs themselves.
Each port can be configured to transmit local information, receive neighbor
information, or both. LLDP transmits information as packets called LLDP
Data Units (LLDPDUs). An LLDPDU consists of a set of Type-Length-
Value elements (TLV), each of which contains a particular type of
information about the device or port transmitting it.
A single LLDPDU contains multiple TLVs. TLVs are short information
elements that communicate complex data, such as variable length strings,
in a standardized format. Each TLV advertises a single type of
information, such as its device ID, type, or management addresses.
The TLVs are grouped as follows
“Mandatory LLDP TLVs” on page 1021
“Optional LLDP TLVs” on page 1021
“Optional LLDP-MED TLVs” on page 1023