CDP and LLDP Neighbor Discovery

CDP and LLDP are Layer 2 protocols that let a device advertise its identity to whatever is directly connected on each port. The scope word that matters is directly connected: discovery does not map the whole network, only your immediate neighbours, one hop away. That is exactly what you want when documenting cabling, confirming which switch an uplink lands on, or catching a cable plugged into the wrong port — and it is why discovery is not a substitute for a topology diagram.

• Device ID or name of the neighbour
• Your local interface and the neighbour's remote port
• Platform and capabilities — is it a switch, a router, a phone?
• In detail output, the neighbour's management IP address and software version

CDP works out of the box between Cisco devices, which is why you have already met it twice this week without configuring anything: it is what logs a native-VLAN mismatch on a trunk, and what tells an IP phone its voice VLAN. LLDP is the vendor-neutral equivalent you switch on when Cisco and non-Cisco gear — or a multi-vendor phone via LLDP-MED — need to discover each other.

SW1# show cdp neighbors
Capability Codes: R - Router, S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone

Device ID    Local Intrfce    Holdtme    Capability   Platform    Port ID
SW2          Gig 0/1          145        S            WS-C2960    Gig 0/1
R1           Gig 0/24         160        R            ISR4331     Gig 0/0/0
SEP0011AABB  Fas 0/10         132        H P          IP Phone    Port 1

Two columns are routinely misread. Local Intrfce is your port — the one on the switch you are logged into. Port ID is the neighbour's port at the far end. Reading them the wrong way round is how cabling documentation ends up backwards. The Capability column is a quick sanity check: an S where you expected an R means the cable does not go where you thought.

The brief table deliberately omits addresses and versions. show cdp neighbors detail adds the neighbour's management IP address and its IOS version, which is what you need before you can log into it or check whether it is running the software you expect. show lldp neighbors and show lldp neighbors detail give the same two views across mixed-vendor gear.

• CDP and LLDP advertise identity to directly connected neighbours only — one hop, never the whole network.
• CDP is Cisco proprietary and on by default; LLDP is the open IEEE 802.1AB standard and often needs lldp run.
• show cdp neighbors gives device ID, your local port, the neighbour's port, and platform; detail adds IP and IOS version.
• Local Intrfce is your port; Port ID is theirs.
• Disable discovery on untrusted and edge ports with no cdp enable so it does not leak topology.