STP 4: Port Priority Tie-Break on Parallel Links

Bundle lab

Guide Rapid-PVST+ to prefer a specific parallel trunk by tuning the sender’s port priority on the root bridge. Two ioll2-xe switches (SW1, SW2) form a physical loop via two equal-speed trunks. A third L2 switch (SW3) extends the user VLAN to a second closet. One Alpine host attaches to SW1 and another to SW3 in VLAN 40 (10.1.40.0/24). You will: force SW1 to be the root for VLAN 40, lower the port priority on SW1’s Gi0/2 (Ethernet0/1) to break the tie so SW2 selects its Gi0/2 as the Root Port, enable PortFast and BPDU Guard on host-facing ports, and verify with show spanning-tree outputs and host pings.

Learning objectives

• Construct a genuine Layer-2 loop with two parallel inter-switch trunks and verify STP blocking.
• Deterministically elect the root bridge by setting a specific bridge priority for VLAN 40.
• Resolve a root-port tie on equal-cost parallel links using the sender’s port priority on the root bridge.
• Enable Rapid-PVST+ on all switches and confirm per-VLAN operation.
• Apply PortFast and BPDU Guard on host-facing access ports only, never on trunks.
• Verify port roles and the winning path using show spanning-tree outputs and end-host pings.

Troubleshooting focus

• If both parallel links forward on SW2, confirm you are running Rapid-PVST+ and that SW1 truly holds the lowest bridge priority for VLAN 40.
• If the wrong SW2 port becomes Root Port, check the sender’s (SW1) port priority on each parallel trunk; the lower sender Port ID wins the tie.
• If hosts cannot ping, validate access VLAN assignment on host-facing ports and that trunks allow VLAN 40 with a consistent native VLAN.
• If an access port stops forwarding after a loop attempt, check BPDU Guard status; receiving BPDUs on an edge port can err-disable it.