Switching Loops and STP Basics

November 2001 • Reading time: 5 minutes

Switching loops create broadcast storms, MAC table instability, and multiple frame copies, making them a severe problem in campus networks. In early deployments, before Spanning Tree Protocol (STP) becomes properly tuned, loops often arise from misconfigured trunk ports or unmanaged switches.

STP steps in by logically disabling ports to create a loop-free topology, even when multiple redundant paths exist physically. Each switch elects a root bridge using the lowest bridge ID, then calculates the shortest path to that root. Non-forwarding ports block frames but still listen to BPDUs, allowing rapid recovery if a primary path fails.

Practical Configuration

• Set bridge priority explicitly to control root bridge election: spanning-tree vlan 1 priority 4096
• Enable portfast only on edge ports to avoid unnecessary TCNs.
• Use BPDU Guard to protect against rogue switch connections.

On Catalyst 5500 and similar platforms, tuning STP timers and enabling features like UplinkFast or BackboneFast can enhance convergence. However, these require testing in lab environments before production rollout.

Eduardo Wnorowski
A networking enthusiast sharing early insights from the field.

Understanding VLANs: Concepts and Configuration Tips

July 2001 · 5 min read

Virtual LANs (VLANs) allow for logical segmentation of Layer 2 networks without requiring physical separation. This provides flexibility, improves security, and enhances network performance.

What is a VLAN?

A VLAN is a broadcast domain created by switches. Instead of grouping devices based on physical location, VLANs enable you to group them logically based on function, department, or application. This decouples network segmentation from physical cabling.

Benefits of VLANs

• Improved network performance through reduced broadcast domains
• Enhanced security by isolating traffic between departments
• Flexibility to move devices without changing physical topology

Basic VLAN Configuration

Here’s a quick look at how to create and assign VLANs on a Cisco Catalyst switch:

Switch# configure terminal
Switch(config)# vlan 10
Switch(config-vlan)# name HR
Switch(config)# interface fa0/1
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 10
  

Remember to assign the correct VLAN to the appropriate interfaces and to use trunking if you want to allow multiple VLANs to traverse a single uplink.

Common Pitfalls

• Forgetting to add the VLAN to the trunk port
• Assigning interfaces to VLANs not configured on the switch
• Using native VLAN mismatches on trunk links

Eduardo Wnorowski is a network infrastructure consultant.
He writes about foundational networking concepts, especially those relevant to campus and enterprise switching.

Recovering Passwords on Cisco Catalyst Switches

March 2001 · 4 min read

Recovering a lost or forgotten password on a Cisco Catalyst switch can be stressful, especially in production environments. Fortunately, Cisco has built-in mechanisms that let engineers regain access without wiping the device entirely.

Accessing ROMMON Mode

For most Catalyst switches in 2001 (such as 2900XL or 3500XL), the first step is to interrupt the boot sequence by sending a break signal during startup. This brings the device into ROMMON or switch: prompt, depending on the model.

Changing the Configuration Register

Once at the prompt, change the configuration register to bypass the startup configuration:

  switch: flash_init
  switch: load_helper
  switch: rename flash:config.text flash:config.old
  switch: boot
  

After the device boots, enter privileged mode, then copy the old configuration file back and reset the password.

Final Steps

Make sure to restore the original config and reset the configuration register to its default (usually 0x2102) before reloading:

  Switch# copy flash:config.old system:running-config
  Switch# configure terminal
  Switch(config)# enable secret 
  Switch(config)# config-register 0x2102
  Switch# write memory
  Switch# reload
  

Password recovery doesn’t have to be painful — just be prepared and know your hardware platform.