LLDP (Link Layer Discovery Protocol)

LLDP discovers directly connected network devices - shows what’s connected to each interface without needing network diagrams. Works across vendors (unlike Cisco’s proprietary CDP).

Enable LLDP Globally

LLDP is disabled by default on many devices.

Switch(config)# lldp run

Disable LLDP Globally

Switch(config)# no lldp run

Enable/Disable LLDP Per Interface

Disable LLDP on specific interface:

Switch(config)# interface GigabitEthernet0/1
Switch(config-if)# no lldp transmit
Switch(config-if)# no lldp receive

Re-enable LLDP on interface:

Switch(config-if)# lldp transmit
Switch(config-if)# lldp receive

View LLDP Neighbors

Shows all directly connected devices.

Switch# show lldp neighbors

Typical output:

Capability codes:
    (R) Router, (B) Bridge, (T) Telephone, (C) DOCSIS Cable Device
    (W) WLAN Access Point, (P) Repeater, (S) Station, (O) Other

Device ID           Local Intf     Hold-time  Capability      Port ID
SW1                 Gig0/1         120        B               Gig0/1
Main_L3             Gig1/1         120        R               Gig1/0/2
Edge                Gig2/1         120        R               Gig0/0/1

Total entries displayed: 3

View Detailed LLDP Neighbor Information

Shows full details including IP addresses, VLANs, capabilities.

Switch# show lldp neighbors detail

Typical output:

Local Intf: Gig0/1
Chassis id: 0cd9.96e8.8a00
Port id: Gig0/1
Port Description: Connection to SW_Core
System Name: SW1
System Description: Cisco IOS Software, Version 12.2(25r)

System Capabilities: B
Enabled Capabilities: B
Management Addresses:
    IP: 10.30.0.4
Auto Negotiation - supported, enabled
Physical media capabilities:
    1000baseT(FD)
    100base-TX(FD)
    100base-TX(HD)
    10base-T(FD)
    10base-T(HD)
Media Attachment Unit type: 30
Vlan ID: 1

Total entries displayed: 1

View LLDP Information for Specific Interface

Switch# show lldp neighbors GigabitEthernet0/1

Or for detailed info:

Switch# show lldp neighbors GigabitEthernet0/1 detail

View LLDP Statistics

Shows LLDP packets transmitted/received and errors.

Switch# show lldp traffic

Typical output:

LLDP traffic statistics:
    Total frames out: 3847
    Total entries aged: 0
    Total frames in: 3842
    Total frames received in error: 0
    Total frames discarded: 0
    Total TLVs discarded: 0
    Total TLVs unrecognized: 0

View LLDP Configuration

Shows global LLDP settings (timers, hold-time, etc.).

Switch# show lldp

Typical output:

Global LLDP Information:
    Status: ACTIVE
    LLDP advertisements are sent every 30 seconds
    LLDP hold time advertised is 120 seconds
    LLDP interface reinitialisation delay is 2 seconds

Configure LLDP Timers

Change advertisement interval (default 30 seconds):

Switch(config)# lldp timer 60

Change hold-time (how long neighbor info is kept, default 120 seconds):

Switch(config)# lldp holdtime 180

Change reinitialization delay (default 2 seconds):

Switch(config)# lldp reinit 5

LLDP vs CDP

Feature	LLDP	CDP
Standard	IEEE 802.1AB (industry standard)	Cisco proprietary
Vendor support	Multi-vendor	Cisco only
Default state	Often disabled	Enabled on Cisco devices
Commands	show lldp	show cdp

Enable both for maximum compatibility:

Switch(config)# lldp run
Switch(config)# cdp run

Practical Use Case: Network Documentation

Step 1: Connect to device via SSH

laptop> ssh admin@10.30.0.2
Password: WELkom@1

SW_Core>

Step 2: Enter privileged mode

SW_Core> enable
Password: WELkom@1
SW_Core#

Step 3: View all neighbors

SW_Core# show lldp neighbors

Device ID           Local Intf     Hold-time  Capability      Port ID
SW3                 Gig0/1         120        B               Gig0/1
SW2                 Gig1/1         120        B               Gig0/1
SW1                 Gig2/1         120        B               Gig0/1
Main_L3             Gig3/1         120        R               Gig1/0/2

Step 4: Get detailed info for specific connection

SW_Core# show lldp neighbors Gig0/1 detail

Local Intf: Gig0/1
System Name: SW3
Management Addresses:
    IP: 10.30.0.6
Port id: Gig0/1

Step 5: Verify interface is trunk or access

LLDP doesn’t show trunk/access mode - use this command:

SW_Core# show interfaces Gig0/1 switchport

Name: Gig0/1
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Voice VLAN: none
Trunking VLANs Enabled: 20,21,22,23,30
Pruning VLANs Enabled: 2-1001

Verify LLDP Status on Interface

Check if LLDP is enabled on specific interface.

Switch# show lldp interface GigabitEthernet0/1

Typical output:

GigabitEthernet0/1:
    Tx: enabled
    Rx: enabled
    Tx state: IDLE
    Rx state: WAIT FOR FRAME

Common LLDP Troubleshooting

Neighbor not appearing?

• Verify LLDP is enabled globally: show lldp
• Check interface LLDP status: show lldp interface
• Ensure neighbor device has LLDP enabled
• Check physical connectivity

Clear LLDP table (force rediscovery):

Switch# clear lldp table

Complete Network Discovery Workflow

Inventory all devices in network:

1. Connect laptop to management VLAN (VLAN 30)
2. SSH to core switch
3. Run show lldp neighbors to see connected devices
4. For each neighbor, SSH to that device
5. Run show lldp neighbors on neighbor
6. Verify trunk/access with show interfaces trunk or show interfaces [interface] switchport
7. Document in table: Device, IP, Interface, Connected Device, Connected Interface, Trunk/Access
8. Repeat for all devices

Example documentation table:

Device	IP Address	Interface	Connected To	Remote Interface	Mode
SW_Core	10.30.0.2	Gig0/1	SW3	Gig0/1	Trunk
SW_Core	10.30.0.2	Gig1/1	SW2	Gig0/1	Trunk
SW_Core	10.30.0.2	Gig2/1	SW1	Gig0/1	Trunk
SW_Core	10.30.0.2	Gig3/1	Main_L3	Gig1/0/2	Trunk