The IEEE 802.1AB Link Layer Discovery Protocol (LLDP) enhances the ability of network
management tools to discover and maintain accurate network topologies and simplify LAN
troubleshooting in multi-vendor environments. To efficiently and effectively operate the various
devices in a LAN you must ensure the correct and valid configuration of the protocols and
applications that are enabled on these devices. With Layer 2 networks expanding dramatically, it is
difficult for a network administrator to statically monitor and configure each device in the network.

Using LLDP, network devices such as routers and switches advertise information about themselves
to other network devices and store the information they discover. Details such as device
configuration, device capabilities, and device identification are advertised. LLDP defines the
following:

•

A common set of advertisement messages.

•

A protocol for transmitting the advertisements.

•

A method for storing the information contained in received advertisements.

NOTE

LLDP runs over the data-link layer which allows two devices running different network layer protocols
to learn about each other.

LLDP information is transmitted periodically and stored for a finite period. Every time a device
receives an LLDP advertisement frame, it stores the information and initializes a timer. If the timer
reaches the time to live (TTL) value, the LLDP device deletes the stored information ensuring that
only valid and current LLDP information is stored in network devices and is available to network
management systems.