Understanding Neighbor Discovery

The neighbor discovery protocols support the Layer 2 process of network devices advertising their identities and capabilities on a LAN and discovering that information about their directly connected neighbors. While Enterasys Discovery Protocol and Cisco Discovery Protocol are vendor-specific protocols, LLDP is an industry standard (IEEE 802.1AB), vendor-neutral protocol.

The LLDP-enabled device periodically advertises information about itself (such as management address, capabilities, media-specific configuration information) in an LLDPDU (Link Layer Discovery Protocol Data Unit), which is sent in a single 802.3 Ethernet frame (see Frame Format). An LLDPDU consists of a set of TLV (type, length, and value) attributes. The information, which is extracted and tabulated by an LLDP-enabled device‘s peers, is recorded in IEEE-defined management information base (MIB) modules, making it possible for the information to be accessed by a network management system using a management protocol such as SNMP. The information is aged to ensure that it is kept up to date. Ports can be configured to send this information, receive this information, or both.

The LLDP agent operates only in an advertising mode, and hence does not support any means for soliciting information or keeping state between two LLDP entities.

LLDP can be used for many advanced features in a VoIP network environment. These features include basic configuration, network policy configuration, location identification (including for Emergency Call Service/E911), Power over Ethernet management, and inventory management.

To fulfill these needs, the standard provides extensions to IEEE 802.1AB that are specific to the requirements of media endpoint devices in an IEEE 802 LAN. Interaction behavior between the media endpoint devices and the LAN infrastructure elements are also described where they are relevant to correct operation or multi-vendor interoperability. Media endpoint devices addressed include, but are not limited to, IP phones, IP voice/media gateways, IP media servers, and IP communication controllers.

The S- K- and 7100-Series device supports a neighbor warning detection feature which enables protocol checking for a set of potential misconfigurations between this device and the neighbor port.

Communication between LLDP-enabled Devices shows an example of LLDP communication between devices, done via Layer 2 with LLDPDU packets. The communication is only between LLDP-enabled devices — the information is not forwarded to other devices.

Communication between LLDP-enabled Devices