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CDP / LLDPInterface Config

lldp transmit

Enables LLDP transmission on an interface, allowing the device to advertise its identity and capabilities to neighboring LLDP-enabled devices.

Syntax·Interface Config
lldp transmit

When to Use This Command

  • Enable LLDP transmit on a switch port connected to a VoIP phone to advertise VLAN information and PoE capabilities.
  • Configure LLDP transmit on a router interface to allow a network management system to discover the device topology.
  • Enable LLDP transmit on an access port to provide neighboring switches with device identification and management IP.
  • Use in conjunction with 'lldp receive' to enable full LLDP operation for network discovery and troubleshooting.

Command Examples

Enable LLDP transmit on an interface

interface GigabitEthernet0/1 lldp transmit
Switch(config-if)# lldp transmit
Switch(config-if)#

The command is entered in interface configuration mode. No output is displayed upon successful execution; the prompt returns without error.

Verify LLDP transmit is enabled

show lldp interface GigabitEthernet0/1
GigabitEthernet0/1:
  Tx: enabled
  Rx: disabled
  Tx state: IDLE
  Rx state: IDLE

The output shows that LLDP transmit is enabled (Tx: enabled) and receive is disabled (Rx: disabled). Tx state and Rx state indicate the current LLDP state machine status.

Understanding the Output

The command 'lldp transmit' itself produces no output when successful. To verify, use 'show lldp interface <interface>'. The key field is 'Tx: enabled' indicating LLDP transmission is active. If 'Tx: disabled' appears, LLDP transmit is not enabled. The 'Tx state' field shows the internal state machine (IDLE, TRANSMITTING, etc.) and is typically IDLE when not actively sending frames. 'Rx state' is irrelevant for transmit-only configuration. In a real network, ensure both 'Tx' and 'Rx' are enabled for full LLDP operation unless only one direction is needed.

CCNA Exam Tips

1.

CCNA exam tip: 'lldp transmit' is configured per interface; it does not enable LLDP globally. You must also enable 'lldp run' globally first.

2.

CCNA exam tip: LLDP is a vendor-neutral protocol; Cisco also has CDP which is proprietary. Know the difference and when to use each.

3.

CCNA exam tip: The 'lldp transmit' command can be used independently of 'lldp receive'; you can enable only one direction.

4.

CCNA exam tip: On some Cisco platforms, LLDP is disabled by default; you must enable it globally with 'lldp run' before interface-level commands take effect.

Common Mistakes

Mistake: Forgetting to enable LLDP globally with 'lldp run' before configuring 'lldp transmit' on an interface. Consequence: The interface command is accepted but LLDP does not operate.

Mistake: Enabling 'lldp transmit' but not 'lldp receive' when full neighbor discovery is needed. Consequence: The device advertises itself but cannot learn about neighbors.

Mistake: Applying 'lldp transmit' to a trunk port without considering that LLDP may carry VLAN information that could conflict with CDP. Consequence: Potential misconfiguration if both protocols are used.

Related Commands

lldp receive

Enables LLDP reception on an interface, allowing the device to receive LLDP advertisements from neighboring devices for network discovery and topology mapping.

lldp run

Globally enables Link Layer Discovery Protocol (LLDP) on the switch, allowing it to advertise and receive device information from directly connected LLDP-capable neighbors.

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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