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IEEE 802.1D) can be used to detect and disable network loops, and to provide link
backup. This allows the switch to interact with other bridging devices (including
STA-compliant switches, bridges or routers) in your network to ensure that only one
route exists between any two stations on the network. If redundant paths or loops are
detected, one or more ports are put into a blocking state (stopped from forwarding
packets) to eliminate the extra paths. Moreover, if one or more of the paths in a stable
spanning tree topology fail, this algorithm will automatically change ports from blocking
state to forwarding state to reestablish contact with all network stations.
STA uses a distributed algorithm to select a bridging device (STA-compliant switch,
bridge or router) that serves as the root of the spanning tree network. It selects a root
port on each bridging device (except for the root device) which incurs the lowest path
cost when forwarding a packet from that device to the root device. Then it selects a
designated bridging device from each LAN which incurs the lowest path cost when
forwarding a packet from that LAN to the root device. All ports connected to designated
bridging devices are assigned as designated ports. After determining the lowest cost
spanning tree, it enables all root ports and designated ports, and disables all other ports.
Network packets are therefore only forwarded between root ports and designated ports,
eliminating any possible network loops.
Once a stable network topology has been established, all bridges listen for Hello BPDUs
(Bridge Protocol Data Units) transmitted from the Root Bridge. If a bridge does not get a
Hello BPDU after a predefined interval (Maximum Age), the bridge assumes that the link
to the Root Bridge is down. This bridge will then initiate negotiations with other bridges
to reconfigure the network to reestablish a valid network topology.
The following figure gives an illustration of how the Spanning Tree Algorithm assigns
bridging device ports.