PlantUML DiagramA Simple Two-Switch Loop
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Monday morning, 9:04. Users on three floors report the network is 'dead'. Your monitoring shows every uplink at 100% utilization and every access switch CPU pinned at 99%. You haven't changed anything in a week. Someone in facilities says they plugged in a new switch in a closet over the weekend.
That story ends one of two ways: you find the loop in fifteen minutes, or the whole floor stays down until you trace every cable. Layer 2 loops are the single most destructive failure mode in a campus network — worse than a dead router, worse than a bad config push, because they don't just stop working, they actively melt your infrastructure.
Ethernet frames have no TTL. IPv4 decrements TTL on each hop and drops the packet at zero. Ethernet has nothing equivalent. A frame forwarded into a loop keeps circulating indefinitely, multiplied by every switch that touches it.
Two switches, two cables. Without STP, every broadcast from Host 1 is flooded out both A→B links. Switch B receives two copies, floods each out both ports to A. Now there are four copies. Then eight. Exponential growth on the wire in under a second.
The MAC table instability symptom is the tell for diagnosis. If `show mac address-table` shows one MAC bouncing between Gi0/1 and Gi0/2 every few seconds, you have a loop. The switch keeps learning the source MAC on whichever port the latest copy arrived on, and the entry flips constantly.
The same MAC on two different ports. Under normal operation, one MAC lives on one port. If you see this, something has connected your switch fabric to itself — accidentally or maliciously. STP prevents this by blocking one of the redundant paths.
Spanning Tree consuming 80%+ of CPU is itself a symptom — STP is constantly recomputing topology because the loop keeps changing the port states. On switches without STP enabled, ARP input and forwarding processes absorb the load instead.
To break a storm, disconnect a link. Any link. The loop is a cycle; cutting any edge in the cycle breaks the broadcast storm. You lose a path but regain the network. Then find the actual redundant link and either shut it or enable STP on it.
The next lesson explains the protocol that prevents all of this. STP isn't glamorous, but understanding it is the difference between a network engineer and someone who reads logs.
Finish the lesson once you have worked through the material. This awards ★ 30 XP.