The provided interface configuration includes the commands lldp transmit and lldp

receive. These commands enable the Link Layer Discovery Protocol (LLDP) on the

interface, which is the appropriate vendor-neutral standard for discovering third-party

devices. However, for these interface-level commands to be active, LLDP must first be

enabled globally on the router. The prompt's statement that "Neighbor discovery is

disabled" indicates that the global configuration command, lldp run, has not been

issued. Therefore, enabling LLDP globally is the necessary action to make the existing

interface configuration functional and complete.

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•

B. Disable autonegotiation: While it is a best practice to disable autonegotiation

(no negotiation auto) when speed and duplex are manually configured to prevent

physical layer issues, it does not address the core issue of enabling the neighbor

discovery protocol mentioned in the prompt.

•

C. Disable Cisco Discovery Protocol on the interface: Disabling the

proprietary Cisco Discovery Protocol (CDP) on an ISP-facing link is a crucial

security best practice. However, this action hardens the interface; it doesn't

enable the intended LLDP functionality, which is the primary task needed to

"complete the configuration" for neighbor discovery.

•

D. Enable LLDP-MED on the ISP device: LLDP-MED (Media Endpoint

Discovery) is an extension for voice and video endpoints (e.g., IP phones), not

for router-to-router links. Furthermore, the configuration would be on the ISP's

equipment, which the engineer does not control.

•

Cisco IOS XE Software Configuration Guide: "By default, LLDP is disabled

globally. To use LLDP on the device, you must enable it globally by using the lldp

run command... After you enable LLDP globally, all interfaces that support LLDP

are automatically enabled for both transmitting and receiving LLDP packets."

This confirms that lldp run is a prerequisite for the interface commands to

function.

o Source: Cisco, "Configuring LLDP, LLDP-MED, and Wired Location,"

Cisco IOS XE Gibraltar 16.12.x (Catalyst 9500 Switches). URL:

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9500/software/

release/16-

12/configuration_guide/if/b_1612_if_config_guide_c9500/configuring_lldp_

lldp_med_and_wired_location.html (Refer to the "Enabling LLDP" section).

•

IETF RFC for LLDP: The standard itself defines the operational state of the

protocol, which device vendors like Cisco implement with global and interface-

level controls.

o Source: IEEE, "IEEE Standard for Local and metropolitan area networks--

Link Layer Discovery Protocol," IEEE 802.1AB-2016. URL:

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https://ieeexplore.ieee.org/document/7433915 (DOI:

10.1109/IEEESTD.2016.7433915).

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