Using CDP and CDPR to Identify Network Devices

November 2000  ·  3 min read

In small or large campus environments, tracking down which switch and port a device is connected to can be a time-consuming task—especially in the absence of good documentation or automated management tools.

This is where Cisco Discovery Protocol (CDP) and its companion utility CDPR come in handy. These tools allow administrators to quickly identify the switch and port to which a device is connected, without physically tracing cables or logging into multiple switches manually.

CDP Basics

CDP is a Layer 2 protocol used to share device information between directly connected Cisco equipment. It provides key insights like device ID, IP address, platform, and the local interface through which the device is connected.

Use the command:

show cdp neighbors detail

to retrieve this information on a Cisco switch or router.

Introducing CDPR

CDPR (Cisco Discovery Protocol Reporter) is an open-source tool that runs on end devices and listens for CDP packets to determine which switch port the device is plugged into. It's especially useful when you don’t have switch access or if you’re working remotely.

Real-World Workflow

• Install and run CDPR on a laptop.
• Plug into a network port in question.
• CDPR will report the device ID and interface of the switch port it is connected to.

This makes it much easier to identify and document port usage during switch audits or troubleshooting tasks.

Security Consideration

Since CDP is a proprietary protocol, it can be disabled in environments that favor strict security postures. But when enabled internally, it becomes a valuable troubleshooting asset.

 

Troubleshooting Spanning Tree Issues in Campus LANs

July 2000 · Reading time: 4 min

Spanning Tree Protocol (STP) plays a crucial role in preventing loops in Layer 2 networks. However, when things go wrong, it can cause widespread disruption across campus LANs. Troubleshooting these problems requires not only a sound understanding of how STP operates, but also a practical approach to isolating issues as they arise.

Understanding the Symptoms

One of the first signs of STP trouble is intermittent connectivity or broadcast storms. These often stem from a failure in topology change propagation or a switch mistakenly taking on the root role due to misconfiguration or link issues.

Basic Steps for Troubleshooting

• Use show spanning-tree to verify the root bridge and port roles.
• Check for topology changes using debug spanning-tree events.
• Ensure portfast is not enabled on uplinks.
• Validate consistency of bridge priorities across the network.
• Use show cdp neighbors to verify physical topology against logical STP view.

Common Misconfigurations

In the mid/late 90s, it was common to find legacy switches in the access layer with default STP priorities, allowing them to compete for root bridge status. Adjusting bridge priorities and enforcing Root Guard on access-facing ports helps protect the STP topology.

Lessons from the Field

In one case, a newly added switch in a student lab area began acting as the STP root after a power cycle. The cause was a default priority and faster boot time. Once the lab switch was pre-configured with a higher bridge priority and Root Guard was enabled on the uplink, stability returned.

 

Managing Collision Domains with Microsegmentation

Published: March 2000 · 5 min read

As campus networks scale, unmanaged collision domains become a significant performance bottleneck. One of the most effective ways to mitigate this issue is through microsegmentation — breaking down large shared domains into smaller, switched segments.

With the widespread adoption of switches like the Catalyst 2900 and 5500 series, we now deploy Layer 2 segmentation using dedicated switch ports per device. This approach eliminates collisions and provides each connected device with a dedicated bandwidth path.

Microsegmentation with VLAN Pruning and Trunking

To prevent broadcast domains from growing uncontrollably, VLANs help isolate traffic. Trunk links carry multiple VLANs between switches, but without pruning, unnecessary broadcast traffic can still flow across the network.

VTP pruning and manual VLAN control on trunks help enforce tighter segmentation. This further reduces the likelihood of collision domain overlap and unnecessary frame propagation.

Best Practices

• Use one switch port per endpoint.
• Segment departments or traffic types with VLANs.
• Enable VTP pruning or manually define VLANs on trunks.
• Monitor interface stats to detect collisions or late errors — they should be zero.

Microsegmentation has become foundational in LAN design, and it’s something we now implement as a standard baseline in any new campus deployment.