A network administrator adds a second uplink between two switches for redundancy. Within seconds of plugging in the cable, all network traffic slows to a crawl and users are disconnected. The admin unplugs the second cable and the network recovers immediately.
What Layer 2 phenomenon occurred, and what protocol should be enabled to prevent it while still providing redundancy?
- A.IP routing loop - enable OSPF to calculate loop-free paths
- B.Duplex mismatch - configure both ends of the link to full-duplex
- C.Layer 2 broadcast storm - the second cable created a loop with no mechanism to break it. Enable STP (Spanning Tree Protocol, IEEE 802.1D) or RSTP (802.1w) to automatically detect the loop and block one of the redundant ports while keeping it available as a failover
- D.ARP table overflow - increase the ARP cache size on both switches
Why C is correct
Without STP, a Layer 2 loop creates a broadcast storm: a single broadcast frame (ARP request, for example) circulates indefinitely through both switches multiplying exponentially until it consumes all bandwidth. Layer 2 frames have no TTL equivalent. STP (802.1D) elects a root bridge and blocks redundant ports (Blocking state), ensuring a loop-free topology while maintaining redundancy. RSTP (802.1w) converges in 1-2 seconds vs STP's 30-50 seconds and is preferred in modern networks.
Know someone studying for Network Fundamentals? Send them this one.