show lldp neighbors – Multi-Vendor Device Discovery with LLDP
What is LLDP (Link Layer Discovery Protocol)?
Definition: LLDP is an IEEE 802.1AB, standards-based, vendor-neutral Layer 2 protocol that enables devices (switches, routers, IP phones, wireless APs, etc.) from any vendor to advertise their identity, capabilities, and management information to directly connected neighbors.
- Promotes interoperability in mixed-vendor environments (unlike Cisco's proprietary CDP).
- Supports automated network inventory, VoIP deployments, and physical topology discovery.
Understanding show lldp neighbors Command
- Usage:
show lldp neighbors - Lists all directly connected LLDP-capable devices and their connecting interfaces—regardless of vendor.
- Enables rapid discovery and documentation in heterogeneous networks.
Sample Output and Fields
Device ID Local Intf Hold-time Capability Port ID HP-Switch01 Gi0/1 120 B, R 1 Polycom-Phone Gi0/2 120 T 4
| Field | Description |
|---|---|
| Device ID | System name/hostname of the neighbor device |
| Local Intf | Your device's interface connected to neighbor (e.g., Gi0/1) |
| Hold-time | Seconds until LLDP info is discarded if not refreshed |
| Capability | Device function (B=Bridge/Switch, R=Router, T=Telephone, etc.) |
| Port ID | Neighbor’s port/interface name or number |
LLDP-MED (Media Endpoint Discovery)
- Extension to LLDP for discovering media devices (IP phones, VoIP endpoints, etc.).
- Advertises device location, VLAN assignments, QoS, power requirements, and more—automates VoIP setup!
-
Example: If John connects a Cisco switch to a Polycom IP phone and runs
show lldp neighbors detail, he’ll see voice VLAN, power settings, and device location—enabling rapid deployment of VoIP phones.
Enabling and Disabling LLDP
Globally (Cisco):configure terminal lldp run # Enable globally no lldp run # Disable globally endPer Interface:
interface GigabitEthernet0/2 lldp transmit # Enable sending LLDP lldp receive # Enable receiving LLDP no lldp transmit # Disable sending LLDP no lldp receive # Disable receiving LLDP end
Note: LLDP is disabled by default on Cisco devices—you must enable it for multi-vendor discovery.
Differences from Cisco’s CDP
| Aspect | LLDP | CDP |
|---|---|---|
| Vendor Support | Multi-vendor (IEEE) | Cisco proprietary |
| Default State (Cisco) | Disabled | Enabled |
| Use Case | Mixed-vendor networks | Cisco-only environments |
| Information Shared | Similar (ID, capabilities, port, some management info) | More Cisco-specific, sometimes richer |
| Security | BOTH expose topology if enabled | BOTH expose topology if enabled |
Security Considerations
- LLDP and CDP both expose detailed topology and device information to any directly connected host—potentially aiding attackers.
- Best Practices:
- Enable LLDP only where needed (infrastructure and uplinks).
- Disable on user-facing or untrusted ports (e.g., access ports, guest networks).
- Regularly audit interfaces running LLDP.
Troubleshooting Using LLDP
- Use LLDP to confirm cabling, port numbers, and cross-vendor device visibility.
- LLDP helps diagnose VLAN mismatches and VoIP assignment issues (with LLDP-MED).
- If a neighbor is missing in
show lldp neighbors: - Is LLDP enabled on both ends?
- Are interfaces up and cabled correctly?
- Run
show lldp(Cisco) orshow lldp info remote-device(HPE/Arista/Juniper, etc.).
Use Cases for LLDP
- Multi-vendor Environments: Discover, document, and troubleshoot networks mixing Cisco, HPE, Juniper, Polycom, Extreme, Arista, etc.
- VoIP Deployments: LLDP-MED automates VLAN and QoS for IP phones—plug and play voice deployments.
- Network Inventory and Mapping: Quickly generate up-to-date topology maps (scripted, manual, or via management software).
Sample Troubleshooting Scenario – Using LLDP
Scenario: John is deploying a new mixed-vendor network. He connects a Cisco switch to an HPE switch but doesn’t see the HPE switch inshow lldp neighbors.Steps:
- Check LLDP status on both devices:
show lldp(Cisco),show lldp info remote-device(HPE). - Enable LLDP if needed (
lldp runglobally,lldp transmit/receiveon interface). - Check that both interfaces are up and properly cabled (
show interfaces status). - Reseat cables if necessary and verify correct ports.
- Test again with
show lldp neighbors—John should now see the HPE switch.
Comparison Table: LLDP vs. CDP Outputs
| Field | show lldp neighbors |
show cdp neighbors |
|---|---|---|
| Protocol | IEEE 802.1AB (vendor-neutral) | Cisco proprietary |
| Device ID | System name/hostname | Hostname (Device ID) |
| Local Interface | Your device’s interface | Your device’s interface |
| Port ID | Remote device’s port/interface | Remote device’s port/interface |
| Capabilities | B=Bridge/Switch, R=Router, T=Telephone, etc. | R=Router, S=Switch, I=IGMP, etc. |
| Hold Time | Seconds until entry is aged out | Same |
| Management Address | Shown in show lldp neighbors detail |
Shown in show cdp neighbors detail |
| Platform/Model | Sometimes shown in detail | Always shown |
| Software Version | Detail output (if advertised) | Detail output |
| Media Extensions | LLDP-MED (VoIP/VLAN/Power/QoS) | Limited (IP phone info via CDP) |
| Vendor Support | All major vendors | Cisco only |
Exam Tips and Key Points
- LLDP is vendor-neutral (IEEE standard) and essential for multi-vendor networks.
- Interpret
show lldp neighborsfields: Device ID, Local/Remote ports, Capabilities. - Enable LLDP globally and per interface (disabled by default on Cisco).
- LLDP-MED is important for VoIP/phones—automates VLAN and policy assignment.
- Compare LLDP vs. CDP—know when to use each and their commands.
- For security, disable LLDP (and CDP) on untrusted/user-facing ports.
- LLDP does NOT run by default on Cisco devices—always enable as needed.