Understand physical ports and local links.
Identify directly connected devices
Neighbour discovery protocols let directly connected devices advertise information such as their name, port and capabilities. Cisco Discovery Protocol (CDP) is Cisco’s protocol. Link Layer Discovery Protocol (LLDP) is an industry standard. These are not routing protocols and do not list every device across the internet.
On supported Cisco IOS devices, try show cdp neighbors and show cdp neighbors detail. LLDP uses show lldp neighbors and show lldp neighbors detail. If LLDP is supported but disabled, lldp run enables it globally. Interface transmit/receive settings also affect what is exchanged.
Identify local and remote interfaces
The local interface belongs to the device where you ran the command. The advertised port ID belongs to the neighbour. If SW1 reports local Gi0/1 and neighbour R1 port Gi0/0, draw that exact connection. A learned management address does not prove it is reachable from your PC.
Apply discovery policy deliberately: advertising internal device details on untrusted links may be undesirable. Simulators support only a subset of commands. Check help and software support rather than interpreting every rejected command as a network fault.
Verification commands
Use these viewing commands to inspect the idea in this lesson. Start in the environment shown above each example.
show cdp neighbors
show lldp neighborsCommand reference · enter in the stated environment; output varies by device.
Practical exercise
Connect two supported switches in your lab. Enable LLDP where supported, wait for advertisements and draw the local-port to remote-port relationship.
Show the answer and explanation
The two device views should describe opposite ends of the same cable. Confirm the cable placement if the discovered ports differ from your drawing.
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Technical references checked on 25 October 2026. These are original teaching explanations, not official Cisco course material. See how content and lab validation are described.
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