Switches and VLANs

LLDP

Also called Link Layer Discovery Protocol, 802.1AB, LLDP-MED, neighbour discovery, neighbor table, voice VLAN

Devices announcing their name, port and addresses to whoever is at the other end of each cable.

Every few seconds (30 by default) each device sends an Ethernet frame of type 0x88cc to a multicast address that switches keep to themselves, holding TLVs: chassis ID, port ID, time to live, then name, description, capabilities and management address. Neighbours keep it in a table for its TTL; it never crosses a second hop. LLDP-MED adds endpoint TLVs, among them the voice VLAN, priority and DSCP a phone should use. The same details are free reconnaissance for anyone on an access port, so announcements are turned off where they serve nothing.

See it on real machines

Learn it on real machines.