CDP / LLDPCCNA 200-301

LLDP Not Enabled — IP Phones Not Being Discovered

Presenting Symptom

IP phones connected to Cisco switches are not being discovered by the network management system, and they fail to receive voice VLAN configuration via LLDP.

Network Context

A small branch office with a Cisco Catalyst 2960-X switch running IOS 15.2 connects to a Cisco 4321 ISR router. The switch has multiple IP phones from Cisco (CP-8841) connected to its FastEthernet ports. LLDP is intended to be used for voice VLAN discovery, but the phones are not receiving the correct VLAN information and remain in the access VLAN.

Diagnostic Steps

Check LLDP global status

show lldp

Global LLDP Information:
  Status: DISABLED
  LLDP advertisements are sent every 30 seconds
  LLDP hold time multiplier: 4
  LLDP reinitialization delay: 2 seconds

If the status shows DISABLED, LLDP is not running globally. This is the root cause. If it shows ENABLED, proceed to check interface-level configuration.

Check LLDP interface status on phone ports

show lldp interface FastEthernet 0/1

LLDP is not enabled on FastEthernet0/1

Even if LLDP is globally enabled, it may be disabled on specific interfaces. The output confirms LLDP is not enabled on the interface where the phone is connected.

Verify CDP status (alternative protocol)

show cdp neighbors

No CDP neighbors detected

If CDP is also disabled, the switch is not advertising itself to the phones. This confirms that no discovery protocol is active.

Check voice VLAN configuration on the interface

show running-config interface FastEthernet 0/1

interface FastEthernet0/1
 switchport mode access
 switchport access vlan 10
 no lldp transmit
 no lldp receive

The interface is configured as an access port without a voice VLAN. LLDP transmit and receive are explicitly disabled. The phone will not receive the voice VLAN information.

Root Cause

LLDP is globally disabled on the switch, and on the specific interfaces, LLDP transmit and receive are also disabled. Without LLDP, IP phones cannot receive the voice VLAN ID (typically VLAN 20) and remain in the default access VLAN (VLAN 10), causing them to fail discovery and not obtain proper voice configuration.

Resolution

Enable LLDP globally and on the phone-facing interfaces, then configure the voice VLAN. 1. Enable LLDP globally: Switch(config)# lldp run 2. Enable LLDP on the interface (if not already enabled): Switch(config)# interface FastEthernet 0/1 Switch(config-if)# lldp transmit Switch(config-if)# lldp receive 3. Configure voice VLAN on the interface: Switch(config-if)# switchport voice vlan 20 4. (Optional) Disable CDP if not needed: Switch(config-if)# no cdp enable

Verification

1. Verify LLDP global status: Switch# show lldp Expected: Status: ENABLED 2. Verify LLDP neighbors: Switch# show lldp neighbors Expected: Device ID: SEPXXXXXXXXXXXX (phone MAC), Local Intf: Fa0/1, Hold time: 120, Capability: T, Port ID: Port 1 3. Verify voice VLAN on interface: Switch# show interfaces FastEthernet 0/1 switchport Expected: Voice VLAN: 20

Prevention

1. Enable LLDP globally on all switches in the network as a best practice for multi-vendor device discovery. 2. Use a standard voice VLAN configuration template for all access ports connecting IP phones. 3. Regularly audit switch configurations to ensure LLDP and voice VLAN settings are applied consistently.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario appears in troubleshooting questions where IP phones are not receiving voice VLAN information. The exam tests the candidate's ability to identify that LLDP is disabled and to configure it correctly. Key fact: LLDP must be enabled globally and on the interface, and the voice VLAN must be configured under the interface.

Exam Tips

Remember that LLDP is disabled by default on Cisco switches; you must issue 'lldp run' globally.

The exam often tests the difference between LLDP and CDP; LLDP is IEEE standard, CDP is Cisco proprietary.

Know that 'show lldp neighbors' is the command to verify LLDP discovery; if no neighbors appear, LLDP is likely disabled.

Commands Used in This Scenario

show interfaces switchport

Displays the administrative and operational status of a switch port, including VLAN membership, trunking mode, and access VLAN, used to verify VLAN configuration and port security settings.

show lldp neighbors

Displays information about directly connected LLDP-capable devices, including device ID, local interface, hold time, capability, and port ID, used to verify LLDP neighbor discovery and troubleshoot Layer 2 connectivity.

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