Switching loop

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Switching loops occur when there is more than one path between two switches in a computer network.

A physical topology that contains switching or bridging loops is necessary for reliability, yet a switched network cannot have loops.

The solution is to allow physical loops, but create a loop free logical topology with the STP (Spanning tree protocol) on the switches.

In the Layer 2 header, there is no Time To Live (TTL) value. If a frame is sent into a Layer 2 looped topology of switches, it can loop forever.

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[edit] Broadcasts

In the case of broadcast packets (Broadcast radiation) over a switching loop the situation can easily spiral out of control into a Broadcast Storm.

[edit] MAC database instability

Switching loops can cause misleading entries in a switch's MAC database and can cause endless unicast frames 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.

[edit] Multiple frame transmissions

In a redundant switched network it is possible for an end device to receive multiple frames.

[edit] Misinterpretations

  1. It is not true that within a switching loop packets will circulate the network until their Time to Live value expires as no TTL exists at Layer 2.


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