LLDP (Link Layer Discovery Protocol) in VyOS

LLDP (Link Layer Discovery Protocol) is a standard data-link layer protocol (IEEE 802.1AB) used to automatically discover neighboring network devices and exchange configuration information. LLDP allows network devices to advertise their identity, capabilities, and neighbors on the local network.

Overview

LLDP is used for:

Automatic network topology discovery: Determining how devices are physically connected
Infrastructure documentation: Automatically collecting information about neighboring devices
Diagnostics: Verifying that cables are connected correctly
Monitoring: Integration with monitoring systems to visualize topology
Inventory: Collecting information about models, software versions, and serial numbers

How LLDP works

LLDP works by transmitting special Ethernet frames (multicast) to the address 01:80:c2:00:00:0e:

The device periodically sends LLDP packets on all active interfaces
Neighboring devices receive these packets and store the information
The information is retained for a specific period (TTL)
Devices also listen for LLDP packets from their neighbors

Information transmitted

LLDP can transmit the following types of data (TLVs - Type-Length-Value):

Chassis ID: Unique device identifier (MAC address, serial number)
Port ID: Port/interface identifier
TTL: Record lifetime (usually 120 seconds)
System Name: Device name (hostname)
System Description: System description (model, OS version)
System Capabilities: Device capabilities (router, switch, bridge)
Management Address: IP address for management
Port Description: Port description
Port VLAN ID: VLAN on the port (optional)
Link Aggregation: Link aggregation information (optional)

LLDP vs CDP

Characteristic	LLDP	CDP (Cisco Discovery Protocol)
Standard	IEEE 802.1AB (open)	Cisco proprietary
Support	All vendors	Cisco only
Multicast address	01:80:c2:00:00:0e	01:00:0c:cc:cc:cc
Send interval	30 seconds (default)	60 seconds
Recommendation	Preferred	For Cisco-only networks

Basic configuration

Enabling LLDP

# Enable LLDP globally
set service lldp

commit
save

Once enabled, LLDP runs on all interfaces by default.

Viewing LLDP neighbors

# Show all neighbors
show lldp neighbors

# Detailed information about neighbors
show lldp neighbors detail

# Neighbors on a specific interface
show lldp neighbors interface eth0

Example output:

Interface    Device ID         Port ID    Capability
---------    ---------         -------    ----------
eth0         switch-core-01    Gi1/0/1    Bridge,Router
eth1         switch-access-02  Gi2/0/24   Bridge

Configuring LLDP parameters

# Enable LLDP
set service lldp

# LLDP packet send interval (in seconds)
set service lldp legacy-protocols cdp  # Optional: for CDP compatibility

# Management address (for accessing the device)
set service lldp management-address 192.168.1.1

commit
save

Advanced configuration

Configuring LLDP on specific interfaces

# Enable LLDP
set service lldp

# Disable LLDP on a specific interface
set service lldp interface eth2 disable

# Re-enable it (remove disable)
delete service lldp interface eth2 disable

commit
save

Configuring a description for neighbors

# Enable LLDP
set service lldp

# Set the location (physical placement)
set service lldp snmp location "DataCenter-1, Rack-15, RU-42"

commit
save

Configuring legacy protocols (CDP)

For compatibility with Cisco devices, you can enable CDP:

# Enable LLDP with CDP support
set service lldp
set service lldp legacy-protocols cdp

commit
save

Note: CDP is a proprietary Cisco protocol, but VyOS can listen for CDP packets for compatibility.

SNMP integration

# Enable LLDP
set service lldp

# Set the SNMP location for LLDP
set service lldp snmp location "Building-A, Floor-3, Room-301"

# Enable SNMP to access the LLDP MIB
set service snmp community public authorization ro
set service snmp listen-address 0.0.0.0

commit
save

The LLDP MIB (LLDP-MIB) is available via SNMP:

Base OID: 1.0.8802.1.1.2
Local information: 1.0.8802.1.1.2.1.3
Remote information: 1.0.8802.1.1.2.1.4

Configuration examples

Example 1: Basic LLDP configuration

# Enable LLDP on all interfaces
set service lldp

# Set the management address
set service lldp management-address 192.168.1.1

# Set the location
set service lldp snmp location "Main Office, Network Closet A"

commit
save

Verification:

show lldp neighbors
show lldp neighbors detail

Example 2: LLDP in an environment with Cisco devices

# Enable LLDP with CDP compatibility
set service lldp
set service lldp legacy-protocols cdp

# Management address
set service lldp management-address 10.0.0.1

commit
save

Verifying Cisco neighbors:

show lldp neighbors
# Or on the Cisco side:
# show cdp neighbors

Example 3: Selectively enabling LLDP

# Enable LLDP globally
set service lldp

# Disable on interfaces facing end users (security)
set service lldp interface eth2 disable
set service lldp interface eth3 disable

# Keep it enabled on uplink interfaces (eth0, eth1)

commit
save

Example 4: LLDP for data center documentation

# Enable LLDP
set service lldp

# Detailed location information
set service lldp snmp location "DC1, Pod-3, Rack-15, RU-42, Row-G"

# Management address
set service lldp management-address 10.255.255.1

# SNMP for monitoring
set service snmp community monitoring authorization ro
set service snmp community monitoring network 10.0.0.0/8

commit
save

Example 5: LLDP with automation (Ansible)

VyOS configuration:

# Enable LLDP
set service lldp
set service lldp management-address 192.168.1.1

commit
save

Ansible playbook for collecting LLDP information:

- name: Collect LLDP information from VyOS routers
  hosts: vyos_routers
  gather_facts: no

  tasks:
    - name: Get LLDP neighbors
      vyos.vyos.vyos_command:
        commands:
          - show lldp neighbors detail
      register: lldp_output

    - name: Display LLDP neighbors
      debug:
        var: lldp_output.stdout_lines

    - name: Save to file
      copy:
        content: "{{ lldp_output.stdout[0] }}"
        dest: "./lldp_{{ inventory_hostname }}.txt"

Example 6: LLDP monitoring with LibreNMS

VyOS configuration:

# LLDP
set service lldp
set service lldp management-address 192.168.1.1

# SNMP for LibreNMS
set service snmp community librenms authorization ro
set service snmp community librenms network 192.168.1.0/24
set service snmp listen-address 0.0.0.0

commit
save

LibreNMS automatically discovers LLDP neighbors and builds a topology map.

Monitoring and diagnostics

Viewing LLDP neighbors

# All neighbors (brief format)
show lldp neighbors

# Detailed information about all neighbors
show lldp neighbors detail

# Neighbors on a specific interface
show lldp neighbors interface eth0

# Information about a specific neighbor
show lldp neighbors interface eth0 detail

Output examples

Brief format:

Capability Codes: R - Router, B - Bridge, W - Wlan r - Repeater, S - Station
                  D - Docsis, T - Telephone, O - Other

Interface    Local    Remote           Remote               Capability
             Port ID  Chassis ID       Port ID
---------    -------  --------------   -------------------  ----------
eth0         eth0     00:1b:21:d4:d0   GigabitEthernet1/0/1 B,R
eth1         eth1     00:1b:21:d4:d1   GigabitEthernet2/0/5 B

Detailed format:

Interface: eth0
  Chassis ID: 00:1b:21:d4:d0:00
  System Name: switch-core-01
  System Description: Cisco IOS Software, Version 15.2(4)E
  Port ID: GigabitEthernet1/0/1
  Port Description: Uplink to VyOS Router
  Capability: Bridge, Router
  Management Address: 192.168.1.10
  Time to Live: 120 seconds

Checking local information

# Local LLDP information
show lldp interface

# Details for a local interface
show lldp interface eth0

Debugging LLDP

# Check that the LLDP daemon is running
show log | match lldp

# Check the lldpd process
ps aux | grep lldpd

# Restart LLDP service (if needed)
sudo systemctl restart lldpd

# Service status
sudo systemctl status lldpd

Monitoring LLDP packets

# Capture LLDP packets on an interface
sudo tcpdump -i eth0 -vv ether proto 0x88cc

# Or using the multicast address
sudo tcpdump -i eth0 ether dst 01:80:c2:00:00:0e

SNMP monitoring of LLDP

# Retrieve LLDP information via SNMP
snmpwalk -v2c -c public 192.168.1.1 LLDP-MIB::lldpRemTable

# Local information
snmpwalk -v2c -c public 192.168.1.1 LLDP-MIB::lldpLocPortTable

# Remote devices
snmpwalk -v2c -c public 192.168.1.1 LLDP-MIB::lldpRemChassisId

Troubleshooting

Problem: LLDP neighbors are not discovered

Diagnosis:

# 1. Check that LLDP is enabled
show configuration service lldp

# 2. Check the interface (whether it is disabled)
show configuration service lldp interface

# 3. Check the interface status
show interfaces

# 4. Capture LLDP traffic
sudo tcpdump -i eth0 ether proto 0x88cc

# 5. Check the logs
show log | match lldp

Solution:

# Enable LLDP if it is disabled
set service lldp

# Make sure the interface is not disabled
delete service lldp interface eth0 disable

# Verify that the interface is up
show interfaces ethernet eth0

commit
save

# Restart the LLDP daemon (if needed)
sudo systemctl restart lldpd

Problem: LLDP works, but the required information is not transmitted

Cause: Some information may not be transmitted by default.

Solution:

# Make sure the management address is set
set service lldp management-address 192.168.1.1

# Set the location
set service lldp snmp location "Building-A, Floor-2"

commit
save

Problem: LLDP does not work with Cisco devices

Cause: Cisco may use CDP instead of LLDP.

Solution:

On VyOS:

# Enable CDP compatibility
set service lldp legacy-protocols cdp

commit
save

On Cisco:

# Enable LLDP (if not enabled)
lldp run

# On the interfaces
interface GigabitEthernet1/0/1
  lldp transmit
  lldp receive

Problem: LLDP neighbors appear and disappear

Cause: The TTL (Time To Live) expires faster than new packets arrive.

Diagnosis:

# Check the TTL in the detailed information
show lldp neighbors detail

# Monitor disappearances
watch -n 5 'show lldp neighbors'

Solution:

The problem is usually on the neighbor’s side. Check:

The LLDP packet send interval on the neighboring device
The CPU load on the neighboring device
Cable or port issues

Problem: LLDP shows incorrect information

Cause: Cached information from an old neighbor.

Solution:

# Restart LLDP to clear the cache
sudo systemctl restart lldpd

# Wait 30-60 seconds to receive new LLDP packets
sleep 60

# Verification
show lldp neighbors

Problem: SNMP does not return LLDP information

Diagnosis:

# Check the SNMP configuration
show configuration service snmp

# Test SNMP access
snmpwalk -v2c -c public localhost SNMPv2-MIB::sysDescr

# Check the LLDP MIB
snmpwalk -v2c -c public localhost LLDP-MIB::lldpLocChassisId

Solution:

# Make sure SNMP is enabled
set service snmp community public authorization ro
set service snmp listen-address 0.0.0.0

commit
save

# Restart SNMP
sudo systemctl restart snmpd

Integration with monitoring systems

LibreNMS

LibreNMS automatically discovers and visualizes LLDP topology.

VyOS configuration:

set service lldp
set service lldp management-address 192.168.1.1
set service snmp community librenms authorization ro
set service snmp listen-address 0.0.0.0

commit
save

In LibreNMS:

Add the device through the Web UI or CLI
LibreNMS automatically discovers LLDP neighbors
The topology map is built automatically (Map → Topology)

Observium

VyOS configuration:

set service lldp
set service lldp management-address 192.168.1.1
set service snmp community observium authorization ro
set service snmp listen-address 0.0.0.0

commit
save

Observium uses the LLDP MIB to build the network map.

Zabbix

VyOS configuration:

set service lldp
set service lldp management-address 192.168.1.1
set service snmp community zabbix authorization ro
set service snmp v3 user zabbix auth type sha
set service snmp v3 user zabbix auth plaintext-password 'AuthPassword123!'
set service snmp v3 user zabbix privacy type aes
set service snmp v3 user zabbix privacy plaintext-password 'PrivPassword123!'

commit
save

The Zabbix template for LLDP monitoring uses SNMP discovery to automatically discover neighbors.

Netbox

Netbox can import LLDP data via the API or scripts.

Script for exporting LLDP to JSON:

#!/bin/bash

# Retrieve LLDP neighbors in JSON format
vtysh -c "show lldp neighbors detail json" > /tmp/lldp_neighbors.json

# Send to Netbox via the API
curl -X POST https://netbox.example.com/api/dcim/cables/ \
  -H "Authorization: Token YOUR_TOKEN" \
  -H "Content-Type: application/json" \
  -d @/tmp/lldp_neighbors.json

Grafana + Prometheus

LLDP data can be exported via an SNMP exporter for Prometheus.

Prometheus SNMP exporter config:

modules:
  lldp:
    walk:
      - 1.0.8802.1.1.2  # LLDP-MIB
    metrics:
      - name: lldpRemSysName
        oid: 1.0.8802.1.1.2.1.4.1.1.9
        type: DisplayString

Best practices

1. Always enable LLDP

Enable LLDP on all network devices for automatic documentation:

set service lldp
set service lldp management-address <mgmt-ip>

2. Set an informative location

Use a detailed description of the physical placement:

set service lldp snmp location "DC-Moscow, Building-2, Floor-3, Rack-A15, RU-42"

3. Disable LLDP on untrusted interfaces

Disable LLDP on ports accessible to end users (security):

set service lldp interface eth2 disable  # User-facing port
set service lldp interface eth3 disable  # Guest network

4. CDP compatibility for mixed environments

In networks with Cisco equipment, enable CDP:

set service lldp legacy-protocols cdp

5. SNMP integration for monitoring

Always configure SNMP to access the LLDP MIB:

set service snmp community monitoring authorization ro

6. Regularly check the topology

Periodically check LLDP neighbors to detect changes:

show lldp neighbors

Use automation (Ansible, Python) for regular audits.

7. Documentation through interface descriptions

Use interface descriptions together with LLDP:

set interfaces ethernet eth0 description "Uplink to switch-core-01 Gi1/0/1"

8. Monitoring topology changes

Configure alerts on LLDP neighbor changes in your monitoring systems to detect:

Unauthorized connections
Equipment replacement
Cable issues

9. Using a management VLAN

Configure the management address from a dedicated management VLAN:

set interfaces ethernet eth0 vif 99 address 10.255.255.1/24
set service lldp management-address 10.255.255.1

10. Backing up LLDP data

Periodically save the LLDP output for documentation:

show lldp neighbors detail > /config/lldp-backup-$(date +%Y%m%d).txt

Useful commands

# Show all LLDP neighbors
show lldp neighbors

# Detailed information
show lldp neighbors detail

# Neighbors on a specific interface
show lldp neighbors interface eth0

# Local LLDP information
show lldp interface

# LLDP configuration
show configuration service lldp

# LLDP logs
show log | match lldp

# Capture LLDP packets
sudo tcpdump -i eth0 ether proto 0x88cc

# SNMP query for LLDP data
snmpwalk -v2c -c public localhost LLDP-MIB::lldpRemTable

# LLDP daemon status
sudo systemctl status lldpd

# Restart LLDP
sudo systemctl restart lldpd

# Save LLDP information to a file
show lldp neighbors detail > /tmp/lldp-neighbors.txt

Integration with automation

Python script for collecting LLDP data

#!/usr/bin/env python3

import subprocess
import json
import re

def get_lldp_neighbors():
    """Retrieve LLDP neighbors from a VyOS router"""
    try:
        # Run the command via SSH or locally
        result = subprocess.run(
            ['vtysh', '-c', 'show lldp neighbors detail'],
            capture_output=True,
            text=True
        )

        # Parse the output
        neighbors = []
        current = {}

        for line in result.stdout.splitlines():
            if line.startswith('Interface:'):
                if current:
                    neighbors.append(current)
                current = {'interface': line.split(':')[1].strip()}
            elif 'System Name:' in line:
                current['system_name'] = line.split(':')[1].strip()
            elif 'Port ID:' in line:
                current['port_id'] = line.split(':')[1].strip()
            elif 'Management Address:' in line:
                current['mgmt_address'] = line.split(':')[1].strip()

        if current:
            neighbors.append(current)

        return neighbors

    except Exception as e:
        print(f"Error: {e}")
        return []

if __name__ == '__main__':
    neighbors = get_lldp_neighbors()
    print(json.dumps(neighbors, indent=2))

Ansible playbook for LLDP auditing

- name: LLDP Topology Audit
  hosts: vyos_routers
  gather_facts: no

  tasks:
    - name: Collect LLDP neighbors
      vyos.vyos.vyos_command:
        commands:
          - show lldp neighbors detail
      register: lldp_data

    - name: Parse and validate topology
      set_fact:
        lldp_neighbors: "{{ lldp_data.stdout[0] | parse_lldp }}"

    - name: Check for unexpected neighbors
      debug:
        msg: "WARNING: Unexpected neighbor detected!"
      when: "'unknown' in lldp_neighbors"

    - name: Generate topology report
      template:
        src: topology_report.j2
        dest: "./reports/{{ inventory_hostname }}_lldp.html"

Conclusion

LLDP (Link Layer Discovery Protocol) is an important tool for automatically discovering and documenting network topology. VyOS provides full LLDP support with additional compatibility for Cisco CDP.

Key benefits of LLDP:

Automatic discovery of neighboring devices
A standardized protocol (IEEE 802.1AB)
Integration with monitoring systems via SNMP
Simplified infrastructure documentation
Diagnostics of connection problems

Recommendations for production:

Enable LLDP on all network devices
Disable LLDP on untrusted ports for security
Use detailed location descriptions
Integrate with monitoring systems (LibreNMS, Zabbix, Observium)
Regularly audit the topology using LLDP data
Document changes in the topology

LLDP in VyOS provides reliable topology discovery and integrates with all modern monitoring systems, making network infrastructure management more efficient.

Reviewed by OpenNix LLC · Last updated on 29 July 2026

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