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Spanning tree — a redundant triangle without a loop
Enterprise Intermediate 5 devices 5 checked tasks 100 points Plan lab
Three switches cabled in a ring. Spanning tree elects a root and blocks exactly one port. Force the root where you want it, then read the blocked port. 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 cabled in a ring. Spanning tree elects a root and blocks exactly one port. Force the root where you want it, then read the blocked port.
Your role: a network engineer on the enterprise team. Level: Intermediate Done when: all 5 goals show as passed (100 points).
Network status
Devices: 2 PCs, 3 switchs, cabled as drawn. No Layer 1 or Layer 2 faults: every link is connected and up. PC1 is a host with 10.30.1.10/24. PC2 is a host with 10.30.1.20/24. SW1, SW2, SW3 start with an empty configuration. Known problems
None: nothing is misconfigured. The network is simply not finished: what the goals list is what you have to build or secure. Goals
Spanning tree blocks exactly one port in the ring (20 pts) All three switches agree on the same root bridge (15 pts) SW1 is the root bridge (priority lowered to 24576) (25 pts) PC1 can still reach PC2 despite the blocked port (25 pts) The access ports to the PCs are edge (portfast) ports (15 pts) The activity, in order
Spanning tree blocks exactly one port in the ring Why: Each goal is a state the lab can check on the running device. Verify with: show running-config All three switches agree on the same root bridge Why: Each goal is a state the lab can check on the running device. Verify with: show running-config SW1 is the root bridge (priority lowered to 24576) Why: The lowest priority wins the root election; the MAC address only breaks ties. Verify with: show spanning-tree PC1 can still reach PC2 despite the blocked port Why: A ping needs a route in BOTH directions. This is the end-to-end proof that everything before it worked. Verify with: ping 10.30.1.20 The access ports to the PCs are edge (portfast) ports Why: PortFast skips the 30-second listening/learning wait for host ports; never use it toward another switch. Verify with: show spanning-tree interface FastEthernet0/1 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. Why this matters
Three switches cabled in a ring. Spanning tree elects a root and blocks exactly one port. Force the root where you want it, then read the blocked port.
What you will be able to do
Spanning tree blocks exactly one port in the ring
All three switches agree on the same root bridge
SW1 is the root bridge (priority lowered to 24576)
PC1 can still reach PC2 despite the blocked port
The network
PC, Switch 8-port — 5 devices in all, already cabled.
Fa0 Fa0/1 Fa0 Fa0/1 Fa0/2 Fa0/2 Fa0/3 Fa0/3 Fa0/2 Fa0/3 PC1 10.30.1.10/24 PC2 10.30.1.20/24 SW1 SW2 SW3 Drawn from this lab: devices, cables, ports and the addresses already set. Device Model Address already set
PC1 PC 10.30.1.10 / 255.255.255.0 PC2 PC 10.30.1.20 / 255.255.255.0 SW1 Switch 8-port configure it SW2 Switch 8-port configure it SW3 Switch 8-port configure it
Cabling
PC1 Fa0 ↔ SW1 Fa0/1 PC2 Fa0 ↔ SW2 Fa0/1 SW1 Fa0/2 ↔ SW2 Fa0/2 SW2 Fa0/3 ↔ SW3 Fa0/3 SW3 Fa0/2 ↔ SW1 Fa0/3
What is configured at the start
Nothing is configured to start with beyond the addresses above: you build the configuration from a clean start.
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 Spanning tree blocks exactly one port in the ring 20 Checked automatically against the device’s live state (blocked). 2 All three switches agree on the same root bridge 15 Checked automatically against the device’s live state (sameRoot). 3 SW1 is the root bridge (priority lowered to 24576) 25 Checked automatically against the device’s live state (stpPriority). and On SW1: show spanning-tree says This bridge is the root . 4 PC1 can still reach PC2 despite the blocked port 25 On PC1: ping 10.30.1.20 must succeed. 5 The access ports to the PCs are edge (portfast) ports 15 On SW1: show spanning-tree interface Fa0/1 portfast. and On SW2: show spanning-tree interface Fa0/1 portfast.
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
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:
SW1 spanning-tree vlan 1 priority 24576
interface f0/1
spanning-tree portfast SW2 interface f0/1
spanning-tree portfast Where this is examined
This lab practises these topics in the zNetLaB certification courses:
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