CDP / LLDPCCNA 200-301

CDP Disabled Globally — No Layer 2 Neighbor Discovery

Presenting Symptom

The network engineer cannot see any CDP neighbors on any directly connected Cisco device, even though physical connectivity is verified.

Network Context

A small branch office with two Cisco Catalyst 2960 switches and one Cisco 4321 router. All devices run IOS 15.x. The switches are connected via trunk links, and the router connects to the ISP. CDP is expected to be enabled by default, but no neighbor information appears.

Diagnostic Steps

Check CDP status on the local device

show cdp

Global CDP information:
	Sending CDP packets every 60 seconds
	Sending a holdtime value of 180 seconds
	CDP is globally disabled

If the output shows 'CDP is globally disabled', CDP is turned off globally. If it shows 'CDP is globally enabled', the issue is elsewhere.

Check CDP neighbors on a specific interface

show cdp neighbors

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
	S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
(no output or blank)

If no neighbors are listed, CDP is either disabled globally or on interfaces. This step confirms the symptom.

Verify CDP is enabled on interfaces

show cdp interface

GigabitEthernet0/0 is administratively down, line protocol is down
	CDP is not enabled on the interface
GigabitEthernet0/1 is up, line protocol is up
	CDP is not enabled on the interface

If all interfaces show 'CDP is not enabled', the global disable is the cause. If some interfaces show enabled, the issue is interface-specific.

Confirm CDP global configuration

show running-config | include cdp

no cdp run

The presence of 'no cdp run' confirms CDP is globally disabled. If the line is absent, CDP is enabled by default.

Root Cause

The global configuration command 'no cdp run' has been applied, disabling CDP on all interfaces. This prevents the device from sending or receiving CDP advertisements, so no neighbors are discovered.

Resolution

Re-enable CDP globally using the following commands in global configuration mode: Router(config)# cdp run This command enables CDP on all interfaces that support it. No per-interface configuration is needed unless CDP was also disabled on specific interfaces with 'no cdp enable'.

Verification

Run 'show cdp' to confirm CDP is globally enabled: Global CDP information: Sending CDP packets every 60 seconds Sending a holdtime value of 180 seconds CDP is globally enabled Then run 'show cdp neighbors' to see neighbors: Device ID Local Intrfce Holdtme Capability Platform Port ID Switch1 Gig 0/0 172 S 2960 Gig 0/1

Prevention

["Avoid using 'no cdp run' unless CDP is intentionally disabled for security reasons. Use interface-level 'no cdp enable' to disable CDP on specific ports only.","Document any global CDP disablement in network change records to avoid confusion during troubleshooting.","Use LLDP as an alternative if CDP is disabled for security, but ensure consistency across the network."]

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify why 'show cdp neighbors' returns no output. The exam tests knowledge that CDP is enabled by default and that 'no cdp run' disables it globally. Candidates must know the difference between global and interface-level CDP configuration.

Exam Tips

Remember that CDP is a Cisco proprietary protocol and is enabled by default on all Cisco devices.

The command 'no cdp run' disables CDP globally, while 'no cdp enable' disables it on a specific interface.

In exam simulations, if 'show cdp neighbors' shows nothing, always check 'show cdp' first to see if CDP is globally disabled.

Commands Used in This Scenario

show cdp neighbors

Displays information about directly connected Cisco devices discovered via CDP, used to verify neighbor relationships and gather device details.

show running-config

Displays the current active configuration in DRAM, showing all non-default settings.

Test Your CCNA Knowledge

Practice with scenario-based questions to prepare for the CCNA 200-301 exam.

Practice CCNA Questions