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Link Layer Discovery Protocol LLDP is a standards-based (IEEE 802.1AD) Layer 2 protocol that allows network devices to advertise their major capabilities on the same LAN segment. This protocol was modeled after proprietary protocols such as Cisco Discovery Protocol (CDP), Extreme Discovery Protocol (EDP), and the little-used Nortel Discovery Protocol (NDP). The information that is distributed is stored by its recipients in a standard Management Information Base (MIB) that can be captured by Simple Network Management Protocol (SNMP) for later analysis or topology creation. LLDP is a one-way protocol, meaning that an LLDP agent can send and receive information, but can never solicit information. Also, LLDP operates over a Layer 2 or Layer 3 interface in untagged mode, which enables the protocol to build a topology regardless of any VLAN configuration. LLDP sends its frames to a link local multicast address of 01-80-C2-00-00-OE, so frames are never relayed to another connected neighbor. Figure 10-4 shows the LLDP frame. Figure 10-4. LLDP frame The LLDP Protocol Data Unit (PDU) contains device capabilities in the form of type length values (TLVs), shown in Figure 10-5. Figure 10-5. LLDP TLVs Here are the four mandatory TLVs: Chassis ID Identifies the chassis end station. This can be identified in several ways, depending on the chassis subtype, but the most common value is the Media Access Control (MAC) address of the transmitting station. 568 | Chapter 10: IP Telephony
