LLDP
Link Layer Discovery Protocol (802.1AB, 2005). Standard L2 protocol where devices announce identity + capabilities to neighbors. Open replacement for Cisco's proprietary CDP. Standard on all modern enterprise switches + servers.
Multicast every 30 sec. TLV (Type-Length-Value) format: system name, port id, system capabilities, management address. LLDP-MED (Media Endpoint Discovery) for VoIP phones (auto-assign VLAN + QoS). Use case: show lldp neighbors maps the network from a switch. Standard in datacenter discovery for auto-config. Server side: Linux lldpd package, Windows registry key for enabling. Security highlight: LLDP leaks topology info — should be disabled toward untrusted ports.