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 radiation 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 loop is necessary for reliability, 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 switches.
Contents |
In the case of broadcast packets (broadcast radiation) over a switching loop, the situation may develop into a broadcast storm.
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. 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.
In a redundant switched network it is possible for an end device to receive the same frame multiple times.
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.