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DTUS065 rev A.7 – June 27, 2014
V.10
Spanning Tree Protocol (STP)
Incentive
Interconnecting various switch devices and MAC bridges in a LAN may
lead to network loops. For example (see picture below), say you have 3
bridges A, B and C, and there is a direct (Ethernet or Wi-Fi) connection
between A and B, another between B and C, another between C and A; then
when a device connected to A sends a broadcast, it will be resent by A to B
and C, B will resend it to C and C will resend it to A. The broadcast frame is
caught in a loop which will soon take a lot of the available bandwidth
resulting in a so-called “broadcast storm”.
However loops may be useful to create backup routes when a link fails. See
“
Point-to-point redundancy with dual band
” section for an example.
Operation
When the STP protocol is activated on several interconnected bridges, they
will exchange information to agree upon a unique way to transmit frames
from one point to another.
The bridges will coordinate to set up a tree structure, thus avoiding loops,
and this tree is capable of rearranging automatically when links are broken.
STP should be activated on all bridges participating in a LAN loop. The
alternate protocol RSTP is an evolution of STP that reacts more rapidly to
broken links in some cases, thus accelerating broken links recovery.
