What three types of traffic are affected by Layer 2 loops, and what is an "unknown unicast" frame?

Broadcasts, multicasts, and unknown unicast frames all loop endlessly without STP (Spanning Tree Protocol).

Traffic types affected:

• Broadcast frames — sent to all ports by design (e.g., ARP (Address Resolution Protocol) requests)
• Multicast frames — typically forwarded like broadcasts by Layer 2 switches
• Unknown unicast frames — the switch doesn't know the destination MAC (Media Access Control), so it floods out all ports except the ingress

Unknown unicast explained: When a switch receives a frame with a destination MAC address that is not in its MAC address table, it has no choice but to forward (flood) the frame out all ports except the one it arrived on. This is the same behavior as a broadcast — and just as vulnerable to loops.

During a loop:

• MAC address table constantly changes as the same source MAC arrives on different ports
• This causes MAC database instability
• High CPU utilization makes the switch unable to forward frames properly
• Hosts caught in the loop become unreachable

Go deeper:

• Unicast flood (Wikipedia) — defines unknown-unicast flooding and why it behaves like broadcast in a loop.
• Forwarding information base (Wikipedia) — the table lookup whose miss triggers the flood.