STP 3: Path Cost — Choose the Forwarding Link (VLAN 30)

Bundle lab

Engineer which trunk forwards by tuning STP path cost in a 3-switch triangle. Force SW1 as the VLAN 30 root at a deterministic priority, enable Rapid-PVST+, harden edge ports with PortFast and BPDU Guard, and raise the STP cost on SW3’s direct uplink to SW1 so SW3 prefers the longer, indirect path via SW2. Verify the resulting root port, alternate (blocked) port, and host reachability across the chosen path.

Learning objectives

• Enable Rapid-PVST+ on all switches and verify RSTP operation
• Deterministically elect SW1 as the VLAN 30 root using bridge priority 4096
• Tune STP path selection by setting an interface-specific VLAN 30 cost on SW3 toward SW1
• Harden access ports with PortFast and BPDU Guard (hosts only, never trunks)
• Verify the selected root port, alternate (blocking) port, and end-to-end host connectivity in VLAN 30

Troubleshooting focus

• If SW1 is not root for VLAN 30, compare bridge priorities and set the lowest (multiple of 4096).
• If the wrong SW3 link forwards, confirm the VLAN-scoped interface cost is applied on the intended port and the value is higher than the competing path.
• If pings fail, inspect trunks for native VLAN alignment and that VLAN 30 is permitted end-to-end.
• If a host loses connectivity after enabling edge features, verify PortFast/BPDU Guard is only on access ports.
• Use show spanning-tree vlan 30 on each switch to check root ID, root path cost, and port roles/states.