Layer 2
CDP / LLDP Neighbor Discovery Lab
Read Cisco CDP and LLDP neighbor output like show cdp neighbors detail. Learn the difference between Device ID, local Interface, and remote Port ID — then rebuild the physical topology from the output.
Layer 2 neighbor discovery
CDP and LLDP reveal directly connected neighbors — not remote routed devices.
- CDP and LLDP operate at Layer 2 and help map physical topology.
- They show Device ID, local interface, remote Port ID, platform, capabilities, and more.
- They do not prove end-to-end IP reachability.
- They may be disabled on untrusted interfaces for security.
CDP
Cisco Discovery Protocol
- Cisco proprietary
- Often enabled by default on Cisco devices
- Shows directly connected Cisco (and some compatible) neighbors
show cdp neighbors
show cdp neighbors detail
# global
cdp run
no cdp run
# per interface
cdp enable
no cdp enable
LLDP
Link Layer Discovery Protocol
- Open standard (IEEE 802.1AB) — vendor-neutral
- Works across multivendor networks
- Same CCNA reading skills: Device ID, local port, remote port
show lldp neighbors
show lldp neighbors detail
# global
lldp run
no lldp run
# per interface
lldp transmit
lldp receive
Reading output — three fields to know
- Device ID — neighbor name (not an interface)
- Interface — your local interface toward that neighbor
- Port ID— the neighbor's outgoing interface back toward you