Gigabit Ethernet Switch Management Guide

DVANCED

OPICS

4-2

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.