Why does Layer 2 Ethernet need STP (Spanning Tree Protocol) while Layer 3 (IP (Internet Protocol)) does not need an equivalent loop-prevention protocol?

Layer 3 has TTL (Time to Live)/Hop Limit fields that kill looping packets, but Layer 2 Ethernet frames have no such mechanism.

Layer 3 loop protection:

• IPv4: TTL — decremented by each router, packet dropped at 0
• IPv6: Hop Limit — same concept, different name

Layer 2 has nothing equivalent. When an Ethernet frame enters a loop, it circulates forever (or until the network crashes). There's no counter, no timeout, no automatic discard.

Consequences of a Layer 2 loop:

• MAC (Media Access Control) address table instability — the table constantly flips as the same MAC appears on different ports
• Link saturation — looping frames consume all bandwidth
• High CPU utilization — switches and end devices are overwhelmed
• Network becomes unusable

STP was developed specifically to fill this gap for Layer 2 Ethernet.

Go deeper:

• IEEE 802.1D (Wikipedia) — the MAC-bridging standard that defines STP precisely because bridged Ethernet has no TTL.
• MAC address table / FIB (Wikipedia) — why a loop destabilizes the very MAC table switching depends on.