Compex WP54AG User Manual
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In short, the main purpose of activating STP is to prevent looping when you
have redundant paths in the network. Without activating STP, redundant
topology will cause broadcast storming.
To establish path redundancy, STP creates a tree that spans all of the devices in
an extended network, forcing redundant paths into a standby, or blocked,
state, but establishing the redundant links as a backup in case the active link
should fail. If STP costs change, or if one network segment in the STP becomes
unreachable, the spanning tree algorithm reconfigures the spanning tree
topology and re-establishes the connection by activating the standby path.
Without spanning tree in place, it is possible that more than one connection
may be simultaneously live, which could result in an endless loop of traffic on
the LAN.
Spanning-Tree Protocol operation is transparent to end stations, which are
unaware whether they are connected to a single LAN segment or a switched
LAN of multiple segments.