lldp transmit

Overview

The `lldp transmit` command, issued in interface configuration mode, enables the transmission of Link Layer Discovery Protocol (LLDP) advertisements on a specific interface. LLDP is a vendor-neutral, IEEE 802.1AB standard protocol that allows network devices to advertise their identity, capabilities, and neighbors on an Ethernet network. By enabling LLDP transmit on an interface, the device sends periodic LLDP frames containing information such as chassis ID, port ID, system name, system description, and supported capabilities (e.g., routing, bridging, telephony).

This command is essential for network discovery, topology mapping, and inventory management, especially in multi-vendor environments where Cisco's proprietary CDP may not be supported. Network engineers reach for `lldp transmit` when they need to ensure that a device advertises itself to LLDP neighbors, typically in conjunction with `lldp receive` to enable bidirectional LLDP operation. The command is often used during initial network deployment, when integrating new devices, or when troubleshooting connectivity issues where neighbor discovery is required.

It fits into a broader workflow of enabling LLDP globally with `lldp run`, then selectively enabling transmit/receive on specific interfaces. Important IOS behavior includes that LLDP is disabled by default globally and on all interfaces; the `lldp run` global command must be issued first. The `lldp transmit` command modifies the running configuration and is saved to startup-config with `copy running-config startup-config`.

There is no privilege level restriction beyond standard user EXEC (privilege level 1 for show commands, but configuration requires privilege level 15). The command takes effect immediately, and LLDP frames are sent at a default interval of 30 seconds (configurable with `lldp timer`). Buffered output is not applicable; the command directly updates the interface configuration.

Understanding this command is critical for CCNA and CCNP candidates as it appears in exam topics on network discovery and troubleshooting. In practice, enabling LLDP transmit on interfaces that connect to other LLDP-enabled devices (switches, routers, wireless LAN controllers, IP phones) populates the neighbor table, which can be viewed with `show lldp neighbors`. This information is invaluable for verifying physical connectivity, detecting unauthorized devices, and automating network management tasks.

The command is also a prerequisite for advanced features like LLDP-MED (Media Endpoint Discovery) for VoIP devices. When compared to CDP, LLDP is preferred in multi-vendor networks, while CDP remains useful in pure Cisco environments. The `lldp transmit` command is the interface-level counterpart to the global `lldp run` command; without it, the interface will not send LLDP frames even if LLDP is globally enabled.

A common mistake is forgetting to enable LLDP globally first, which results in the interface command being accepted but having no effect. Another pitfall is enabling transmit without receive, which prevents the device from learning about neighbors. For CCNA/CCNP candidates, mastering this command involves understanding its interaction with `lldp receive`, `lldp timer`, `lldp holdtime`, and `lldp reinit`.

In summary, `lldp transmit` is a foundational command for network discovery, enabling devices to announce their presence and capabilities to neighboring devices, thereby simplifying network management and troubleshooting.