Switching loop

A Switching loop or Bridge loop occurs in computer networks when there is more than one Layer 2 (OSI model) path between two endpoints (e.g. multiple connections between two network switches or two ports on the same switch connected to each other). The loop creates broadcast storms as broadcasts and multicasts are forwarded by switches out every port, the switch or switches will repeatedly rebroadcast the broadcast messages flooding the network. Since the Layer 2 header does not support a time to live (TTL) value, if a frame is sent into a looped topology, it can loop forever.

A physical topology that contains switching or bridge loops is attractive for redundancy reasons, yet a switched network must not have loops. The solution is to allow physical loops, but create a loop-free logical topology using the spanning tree protocol (STP) on the network switches.

Broadcasts

In the case of broadcast packets (broadcast radiation) over a switching loop, the situation may develop into a broadcast storm.

MAC database instability

Switching loops can cause misleading entries in a switch's media access control (MAC) database and can cause endless unicast frames to be broadcast throughout the network because if there was a single broadcast, the switch "thinks" that on both connected ports is the same host like at a single other port (the "correct" port; this happens because both ports received the broadcast packet with the sender address of the host). In a redundant switched network it is possible for switches to learn the wrong information. A switch can incorrectly learn that a MAC address is on one port, when it is actually on a different port.

Briefly MAC database instability results when multiple copies of a frame arrive on different ports of a switch.

Multiple frame transmissions

In a redundant switched network it is possible for an end device to receive the same frame multiple times.

Misinterpretations

It is not true that within a switching loop packets will circulate the network until their time to live (TTL) value expires, as no TTL concept exists at Layer 2. In practice, the packet will circulate until it is dropped, e.g. due to resource exhaustion.