CDP / LLDPCCNA 200-301

CDPv1 and CDPv2 Version Mismatch Between Devices

Presenting Symptom

A network engineer observes that CDP neighbor information is missing or incomplete between two directly connected Cisco devices, even though both devices are physically connected and operational.

Network Context

The problem occurs in a small branch office network with two Cisco Catalyst 2960 switches connected via a trunk link. One switch runs IOS version 15.0 with CDPv2 enabled by default, while the other runs an older IOS version 12.2 that only supports CDPv1. The engineer notices that 'show cdp neighbors' on either switch does not list the other device.

Diagnostic Steps

Check CDP status on both devices

show cdp

Global CDP information:
    Sending CDP packets every 60 seconds
    Sending a holdtime value of 180 seconds
    Sending CDPv2 advertisements is enabled

If CDPv2 advertisements is enabled on one device but the other only supports CDPv1, they will not exchange CDP information. Look for the line 'Sending CDPv2 advertisements is enabled' or 'disabled'.

Check CDP neighbors on each device

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 empty)

If no neighbors are listed, CDP is not exchanging information. This could be due to version mismatch or CDP being disabled on an interface.

Check CDP interface status

show cdp interface

GigabitEthernet0/1 is up, line protocol is up
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds

Ensure the interface is up and CDP is enabled. If the interface is down or CDP is disabled, that would explain the issue.

Verify CDP version compatibility

show cdp neighbors detail

-------------------------
Device ID: SwitchB
  Platform: cisco WS-C2960-24TT-L,  Capabilities: Switch
  Interface: GigabitEthernet0/1,  Port ID (outgoing port): GigabitEthernet0/1
  Holdtime : 0 sec

Version :
Cisco IOS Software, C2960 Software (C2960-LANBASEK9-M), Version 12.2(55)SE, RELEASE SOFTWARE (fc1)
  advertisement version: 1

Look for 'advertisement version: 1' or '2'. If one device shows version 1 and the other version 2, they will not exchange CDP information. This confirms the version mismatch.

Root Cause

The root cause is a CDP version mismatch between the two switches. One switch (running IOS 15.0) has CDPv2 enabled by default, while the other (running IOS 12.2) only supports CDPv1. CDPv1 and CDPv2 are not interoperable, so they do not exchange neighbor information.

Resolution

To resolve the issue, disable CDPv2 on the newer switch to force it to use CDPv1, or upgrade the older switch to support CDPv2. The commands to disable CDPv2 globally are: Switch(config)# no cdp advertise-v2 This command disables CDPv2 advertisements, making the switch use CDPv1. Alternatively, you can enable CDPv2 on the older switch if it supports it (not applicable here). After applying, CDP neighbor information should appear.

Verification

After applying the fix, run the following commands to verify: show cdp neighbors 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 SwitchB Gig 0/1 150 S WS-C2960-24TT-L Gig 0/1 Also run: show cdp Expected output: Global CDP information: Sending CDP packets every 60 seconds Sending a holdtime value of 180 seconds Sending CDPv2 advertisements is disabled

Prevention

1. Ensure all Cisco devices in the network run IOS versions that support the same CDP version (preferably CDPv2). 2. When integrating older devices, proactively configure CDP version compatibility before deployment. 3. Use network discovery protocols consistently; consider using LLDP as an alternative if multi-vendor interoperability is needed.

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 knowledge of CDP versions and the command 'no cdp advertise-v2'. Candidates must know that CDPv1 and CDPv2 are incompatible and how to verify and change the CDP version.

Exam Tips

Remember that CDPv1 and CDPv2 are not interoperable; if one device uses v2 and the other v1, they won't see each other.

The command 'no cdp advertise-v2' globally disables CDPv2; it can also be applied per interface with 'no cdp enable' to disable CDP entirely.

In the exam, you may be asked to interpret 'show cdp neighbors detail' output to find the advertisement version.

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 cdp neighbors detail

Displays detailed information about directly connected Cisco devices discovered via CDP, including IP addresses, IOS version, platform, and interface details, useful for verifying neighbor relationships and troubleshooting Layer 2 connectivity.

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