Allied Telesis AT-S29 - Manual (Page 119)

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Chapter 4
Advanced Topics
This AT-8324SX switch supports Layer 2 switching and other advanced
features, which are described in this chapter.
Spanning Tree Algorithm
The Spanning Tree Algorithm (that is, the STA configuration algorithm as
outlined in IEEE 802.1 D) 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 re-establish 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.