Neighbor Discovery Overview
13-2 Configuring Neighbor Discovery
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 Figure 13-3 on
page 13-6). 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.
Figure 13-1 on page 13-3 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.
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