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OPICS
Spanning Tree Algorithm
The Spanning Tree Algorithm (that is, the STA configuration
algorithm as outlined in 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.
The 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.
4-2
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