STP 2: Controlling the Root Bridge with Priority

Bundle lab

Beginner CCNA STP lab on a three-switch triangle with two hosts in VLAN 20. You will deliberately control the Spanning Tree root election using the root primary/secondary macros so SW1 is the active root and SW2 is the standby. The baseline already provides VLANs and trunks; your job is to set root priorities deterministically and verify the resulting roles and port states without introducing any Layer-3.

Learning objectives

• Understand how STP elects a root bridge and why deterministic control matters
• Use the spanning-tree vlan <N> root primary/secondary macros to set bridge priority
• Verify root, designated, and non-designated port roles and states in a triangle topology
• Confirm that host connectivity in the user VLAN persists across whichever link STP leaves forwarding
• Cross-check the numeric priorities produced by the macros in the running configuration

Troubleshooting focus

• If SW1 is not the root for VLAN 20, ensure VLAN 20 exists and the trunks actually carry it so SW1 can hear other BPDUs.
• If ports are inconsistent across the triangle, check that only one link blocks and the blocked port is consistent with the final root.
• If hosts cannot ping, verify both access ports are in VLAN 20 and that trunks permit VLAN 20 end-to-end.