A loop free network is one in which no Layer 2 loops exist, therefore the
network cannot create layer 2 broadcast storms or flooded unicast storms. A
loop free network can be achieved manually by shutting down or disconnecting
all redundant links between bridges. However, this leaves no redundancy in the
network and requires manual intervention in the event of a link failure.
STP resolves this problem. Where there are alternate links to a destination
on a switch, only one link will be used to forward data unless there is a
failure on that link. The switch ports associated with alternate paths remain
aware of the topology of the network and can be enabled if a failure occurs on
primary link. In the case of primary link failure, the switch will begin
forwarding frames over an alternate link.
The spanning-tree algorithm
(STA) runs on each switch to activate or block redundant links. To find the
redundant links, the STA chooses a reference point in the network and
determines if there are redundant paths to that reference point. If the STA
finds a redundant path, it chooses which path will forward frames and which
redundant path or paths will be blocked. This effectively severs the redundant
links within the network until they are needed when the primary link toward the
reference point fails.
Spanning tree standards often refers to a
"bridge" but it is likely that all the devices exchanging spanning
tree information will be Layer 2 switches.