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CDP / LLDPCCNA 200-301

CDP Neighbors Not Appearing in show cdp neighbors

Presenting Symptom

The 'show cdp neighbors' command returns no output or only lists the local device, indicating that no CDP neighbors are being discovered.

Network Context

A small branch office network with two Cisco 2960 switches and one Cisco 4331 router. The switches are connected via trunk links, and the router connects to the ISP. All devices run IOS 15.x. CDP is enabled globally by default, but neighbors are not appearing.

Diagnostic Steps

1

Verify CDP is enabled globally

show cdp
Global CDP information:
	Sending CDP packets every 60 seconds
	Holdtime is 180 seconds
	CDP is enabled globally

If CDP is disabled globally, no neighbors will be discovered. Look for 'CDP is enabled globally'. If disabled, enable with 'cdp run'.

2

Check CDP status on specific interfaces

show cdp interface
GigabitEthernet0/0 is up, line protocol is up
	Encapsulation ARPA
	Sending CDP packets every 60 seconds
	Holdtime is 180 seconds
	CDP is enabled on the interface

Ensure CDP is enabled on the interface connecting to the neighbor. If disabled, enable with 'cdp enable' under interface config. Also verify interface is up/up.

3

Check if CDP is disabled on the neighbor device

show cdp neighbors detail
No output or 'Device ID' missing

If no output, the neighbor may have CDP disabled. Alternatively, check if the neighbor is a non-Cisco device (CDP is Cisco proprietary).

4

Verify Layer 1 and Layer 2 connectivity

show interfaces status | include connected
Port      Name   Status       Vlan       Duplex Speed Type
Gi0/1            connected    1          a-full a-100 10/100/1000BaseTX

Ensure the interface is in 'connected' state. If not, check cabling, speed/duplex mismatches, or VLAN misconfigurations.

Root Cause

CDP is disabled on the interface connecting to the neighbor, or CDP is globally disabled on the neighbor device. Alternatively, the neighbor is a non-Cisco device that does not support CDP.

Resolution

1. Enable CDP globally if disabled: 'configure terminal', 'cdp run'. 2. Enable CDP on the specific interface: 'interface GigabitEthernet0/1', 'cdp enable'. 3. If neighbor is non-Cisco, consider using LLDP (Link Layer Discovery Protocol) as an alternative: 'lldp run' globally and 'lldp enable' on interfaces.

Verification

Run 'show cdp neighbors' again. Expected output: 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 Switch-2 Gig 0/1 172 S I WS-C2960 Gig 0/1

Prevention

1. Keep CDP enabled by default on Cisco devices unless security policies require disabling it. 2. Use consistent IOS versions across devices to avoid CDP compatibility issues. 3. Document all non-Cisco devices in the network and plan for LLDP if CDP is not supported.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify why CDP neighbors are not showing. The exam tests your knowledge of CDP configuration, verification commands, and the fact that CDP is Cisco proprietary. Key fact: CDP is enabled by default on Cisco devices, but can be disabled globally or per interface.

Exam Tips

1.

Remember that CDP is enabled by default on Cisco devices; if neighbors are missing, check interface-level CDP enable first.

2.

The exam may present a scenario where CDP is disabled due to security policy; know the commands 'cdp run' and 'cdp enable'.

3.

Be aware that CDP only works with Cisco devices; for multi-vendor environments, LLDP is the standard alternative.

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.

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