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AdvancedPublished 2026-07-02

STP Lab 8: Per-VLAN Load Balancing Across Two Roots

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Use Rapid-PVST+ to elect different root bridges per VLAN across two parallel trunks, spreading VLAN 80 and VLAN 81 across distinct physical links without modifying path cost or port priority. Validate independent per-VLAN trees and confirm hosts in VLAN 80 can communicate over the surviving path. Includes a drift-check to diagnose/restore trunk allow-lists.

Track
Est. time
65 minutes
Difficulty
Advanced
Topics

Learning objectives

  • Create VLANs 80, 81, and set a parking native VLAN 999; place users on VLAN 80 access ports only.
  • Configure two parallel L2 trunks between SW1 and SW2 that carry VLANs 80 and 81 and use native VLAN 999 with negotiation disabled.
  • Enable Rapid-PVST+ and elect deterministic per-VLAN roots: SW1 root for VLAN 80, SW2 root for VLAN 81 (bridge priority 4096 each).
  • Harden host-facing access ports with PortFast and BPDU Guard only on access links.
  • Verify per-VLAN tree independence: show different forwarding/blocked interfaces for VLAN 80 vs VLAN 81 on the same physical links.
  • Diagnose a trunk allow-list drift and native VLAN mismatch, then restore symmetric trunk policy.

Troubleshooting focus

  • If hosts in VLAN 80 cannot ping, confirm both trunks permit VLAN 80 on both ends and that access ports are in VLAN 80.
  • If both VLANs ride the same trunk link, verify per-VLAN root election and confirm trunks are cross-connected and symmetric; then compare the per-VLAN root-port choice on each switch.
  • If a trunk shows blocking for both VLANs on the same side, check for native VLAN mismatch (expect native 999) and ensure DTP is disabled with switchport nonegotiate.
  • If PortFast/BPDU Guard triggers on host ports, ensure they are access ports that do not receive BPDUs and are not cabled to another switch.

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How this lab is graded
  • Build it your way. Where a lab lets you choose a value — a VLAN name, an interface description — grading checks that you configured it, not which name you picked. Names that another line has to reference, like an ACL applied with access-class, are stated in the guide and do have to match.
  • Addresses, modes and protocol keywords are exact. An IP address, a subnet mask, switchport mode trunk, an encapsulation — these carry the meaning of the lab, so they are graded as written in the guide.
  • Grading reads your saved configuration. Export the lab from CML after you have configured it, and make sure anything you set is in the running-config — a change that only exists in a terminal session never reaches the grader.
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