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.
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