The rogue switch
Users on SW3 complain that everything got slow after a colleague "added a few ports" at their desk: they plugged a small switch, ROGUE, into SW3's Fa0/3. SW1 should be the root bridge (priority 24576). Find out what happened, remove the rogue switch (click its cable, then Unplug), and protect Fa0/3 so that a switch plugged in there is shut down at once, while a PC on it still comes up immediately.
How it works: this is VulnTech NetLab, a network simulator running in your browser. Click a device in the diagram to open its console and type commands, just like on real equipment (? shows help, Tab finishes a word). Click a cable to inspect it. Tasks tick themselves off as you complete them; questions are answered in the box under each task. Your work is kept if you reload; Reset starts the network over.
Solution explanation
ROGUE had priority 0 (someone's lab switch), so it won the election and the whole network's tree was rebuilt around a desk switch on a 100 Mb/s port. Unplugging it lets SW1 (24576) win again. PortFast plus BPDU guard on Fa0/3 means a PC still gets the network at once, but any switch that sends a BPDU there is err-disabled before it can affect the tree. Root guard on SW3's access ports would also have stopped ROGUE becoming root.
Show solution
[SW3]
enable
show spanning-tree vlan 1 | include Root ID|Address|Port
@unplug SW3 fa0/3
configure terminal
interface fa0/3
spanning-tree portfast
spanning-tree bpduguard enable
end
show spanning-tree vlan 1 | include Root ID|Address|This