zNetLaB › Guided labs › Enterprise
Spanning tree root 43 — make SW-A the root (Rapid-PVST)
Enterprise Intermediate 3 devices 3 checked tasks 100 points Plan lab
Three switches in a triangle elect a root by the lowest bridge ID, and the oldest MAC wins by default — usually the wrong switch. Make SW-A the root bridge with priority 12288, and run Rapid-PVST on all three for fast convergence. You build it in a browser, type real commands at each device, and every task below is
checked against the running configuration — so you find out it is wrong before anybody else does.
Open this lab in the simulator
The simulator runs in your browser with nothing to install. This lab is part of the full simulator: open it free for 7 days with a new account, then with a plan. It needs an account, which keeps your progress.
The scenario
Three switches in a triangle elect a root by the lowest bridge ID, and the oldest MAC wins by default — usually the wrong switch. Make SW-A the root bridge with priority 12288, and run Rapid-PVST on all three for fast convergence.
Your role: a network engineer on the enterprise team. Level: Intermediate Done when: all 3 goals show as passed (100 points).
Network status
Devices: 3 switchs, cabled as drawn. No Layer 1 or Layer 2 faults: every link is connected and up. SW-A starts with and spanning-tree settings already configured. SW-B starts with and spanning-tree settings already configured. SW-C starts with and spanning-tree settings already configured. Known problems
The network worked and now does not: the scenario above says what users report. The cause is a configuration fault, not a cable. Find it with show commands (interfaces, routes, neighbours, running-config) before you change anything; fix only what is wrong. Goals
SW-A has priority 12288 (or lower) for VLAN 1 (30 pts) SW-A is the root bridge (40 pts) All three switches run Rapid-PVST (30 pts) The activity, in order
SW-A has priority 12288 (or lower) for VLAN 1 Why: The lowest priority wins the root election; the MAC address only breaks ties. Verify with: show spanning-tree SW-A is the root bridge Why: The root decides which links forward and which block. Leave it to chance and the slowest switch can win. Verify with: show spanning-tree All three switches run Rapid-PVST Why: Every switch in a domain must run the same mode or the topology will not converge as designed. Verify with: show spanning-tree summary Tips
Work in the order given: later goals depend on the earlier ones. After every change, run the verify command before moving on; do not guess. Stuck? Read the verify output first. It names the interface, neighbour or route that is wrong. Common mistake: Setting priority to a value that is not a multiple of 4096 Common mistake: Only configuring a backup root Common mistake: Mixing PVST+ and Rapid-PVST and expecting fast convergence Why this matters
Redundant switch links create loops, and Ethernet frames never expire. Spanning tree elects one root bridge and blocks the loops; if you do not choose the root, the oldest switch becomes it — often an old access switch at the edge.
What you will be able to do
Set bridge priority to control the root election
Choose Rapid-PVST and explain why
Read show spanning-tree
The network
Catalyst 2960-24TT — 3 devices in all, already cabled.
Gi0/1 Gi0/1 Gi0/2 Gi0/1 Gi0/2 Gi0/2 SW-A SW-B SW-C Drawn from this lab: devices, cables, ports and the addresses already set. Device Model Address already set
SW-A Catalyst 2960-24TT configure it SW-B Catalyst 2960-24TT configure it SW-C Catalyst 2960-24TT configure it
Cabling
SW-A Gi0/1 ↔ SW-B Gi0/1 SW-B Gi0/2 ↔ SW-C Gi0/1 SW-C Gi0/2 ↔ SW-A Gi0/2
What is configured at the start
These are already configured when the lab opens — including, where this is a troubleshooting lab, the fault itself:
SW-A spanning-tree mode pvst SW-B spanning-tree mode pvst SW-C spanning-tree mode pvst Tasks, and how each one is checked
Every task is decided by reading the live network — a neighbour in FULL, a ping that succeeds, a line in the running configuration — not by looking for a keyword you typed. The same command shows you what the lab sees.
# Task Pts How to verify it yourself
1 SW-A has priority 12288 (or lower) for VLAN 1 30 Checked automatically against the device’s live state (stpPriority). 2 SW-A is the root bridge 40 On SW-A: show spanning-tree says This bridge is the root . 3 All three switches run Rapid-PVST 30 Checked automatically against the device’s live state (stpMode). and Checked automatically against the device’s live state (stpMode). and Checked automatically against the device’s live state (stpMode).
Stuck? Ask the coach
Tell the coach what you see or what you tried. It gives you a hint, not the answer, the way a senior engineer would. Included with a free trial or any plan.
Ask the coach
Common mistakes
Setting priority to a value that is not a multiple of 4096
Only configuring a backup root
Mixing PVST+ and Rapid-PVST and expecting fast convergence
Reference solution
Try it yourself first — the lab checks your work either way. This is one configuration that makes every task pass, device by device:
SW-A spanning-tree vlan 1 priority 12288 SW-A spanning-tree mode rapid-pvst SW-B spanning-tree mode rapid-pvst SW-C spanning-tree mode rapid-pvst Where this is examined
CCNA 2.0 STP; CCNP ENCOR Layer 2.
This lab practises these topics in the zNetLaB certification courses:
Spanning Tree (RSTP/PVST+) — CCNA · Network Access Trunking, EtherChannel, RSTP and MST — CCNP Enterprise · ENCOR · Infrastructure Switched campus: VTPv3, MST, EtherChannel, UDLD, loop guard — CCIE Enterprise Infrastructure · Network Infrastructure VLANs, trunking, STP, EtherChannel, VTP — CompTIA Network+ · Network Implementation Spanning tree: choosing the root — Switching and VLAN Mastery · Loop prevention and bundling
Related labs
Where to start
The lessons all guided labs plans
CCNA®, CCNP® and Cisco® are trademarks of Cisco Systems, Inc. zNetLaB is not affiliated with or endorsed by Cisco.