Segmenting with VLAN Pruning and Trunks

Published: November 1999 • 5 min read

In campus switching environments, segmentation is a key concept to maintain performance and manageability. VLANs (Virtual LANs) are commonly used to divide broadcast domains logically. However, with larger switch deployments, ineffective use of VLANs can lead to unnecessary traffic flooding and spanning-tree convergence issues.

One of the simplest ways to improve this is through VLAN pruning. This ensures that VLAN traffic only traverses trunk links where it's truly needed. Combined with proper trunk configuration and native VLAN awareness, engineers can scale Catalyst switch environments while minimizing broadcast overhead.

Why Prune VLANs?

Without VLAN pruning, all VLANs are allowed over all trunks by default. This behavior can result in traffic being forwarded over segments that have no hosts for that VLAN. This not only wastes bandwidth but increases the potential for loops and unnecessary STP (Spanning Tree Protocol) processing.

By using vtp pruning or manually configuring allowed VLANs on trunks, traffic is constrained to where it's actually needed. This is particularly effective in environments with multiple access layer switches feeding into a collapsed backbone.

Trunk Management

Trunks should be clearly documented and controlled. Use static trunking where possible (switchport mode trunk) and limit VLANs via switchport trunk allowed vlan. Avoid relying solely on dynamic trunking protocols like DTP unless absolutely necessary.

Also, verify consistency of native VLAN configurations across trunk links. Mismatches can lead to spanning tree inconsistencies and forwarding issues. Use show interfaces trunk to audit and correct discrepancies.

 

Understanding Spanning Tree: Basics for Network Engineers

Published: July 1999 · Reading time: 6 minutes

Spanning Tree Protocol (STP) is critical in preventing loops in bridged Ethernet networks. In the late '90s, as switching technology matured and campus networks grew larger, understanding how STP operates became essential for network engineers deploying multi-switch topologies.

What Is Spanning Tree?

Spanning Tree Protocol ensures there are no loops in the Layer 2 domain by blocking redundant paths while maintaining network redundancy. It identifies a root bridge and then calculates the shortest path to the root for all switches.

Key Components

• Root Bridge: The central reference point for the STP topology.
• Bridge Protocol Data Units (BPDUs): Packets exchanged to determine the topology.
• Designated Ports and Blocking Ports: Roles that determine data forwarding paths.

Real-World Application

In practice, STP helps maintain uptime even during failures. In my experience, careful placement of root bridges and manually setting port costs helped maintain stability and failover efficiency in enterprise environments.

 

Catalyst 5500: Scaling Campus Networks

Published: March 1999

 

⏱️ Estimated reading time: 2 minutes

Campus networks demand stability and scalability from their core. In this post, I explain how to use the Catalyst 5500 switch to meet these needs. This modular platform supports a wide range of interfaces and supervisory engines, which makes it ideal for growing enterprises.

I configure VLANs, deploy redundant supervisors, and optimize spanning tree settings to build a reliable Layer 2 backbone. By aggregating multiple Fast Ethernet or Gigabit Ethernet links, I ensure bandwidth meets user and application demands without compromising uptime.

The Catalyst 5500 plays a key role in campus design by serving as a distribution or core layer device. With modular slots, I adapt to evolving interface needs, such as FDDI or ATM, though Fast/Gig uplinks dominate most environments. I maintain configuration backups and document slot assignments for fast recovery and simplified audits.

High availability comes from redundant power supplies and supervisor failover. This allows the switch to recover quickly from hardware issues. I monitor CPU load, interface counters, and backplane utilization to catch performance issues early and stay ahead of demand.

For those new to modular switch design, the 5500 offers a balance of simplicity and power—especially when paired with sound configuration and cabling practices.