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DES-2218 Ethernet Switch User’s Guide
Using the Console Interface
43
Introduction to Spanning Tree Protocol
Parameters
The Spanning Tree Protocol works on two levels: the bridge (i.e., switch) and
port levels. At the bridge level, the STP algorithm calculates a bridge
identifier for each bridge and then determines the root bridge and the
designated bridges. On the port level, the spanning tree protocol determines
the root port and designated ports.
On the bridge level, these terms are used:
♦ Root Bridge The bridge with the lowest-numbered bridge identifier.
The root bridge should be the best bridge among the bridges on the
loop to ensure the highest network reliability and performance.
♦ Designated Bridge A bridge becomes the designated bridge for a
network segment if it has a lower root path cost to the root bridge than
the other bridges on the same segment. If all bridges have the same
root path cost, then the one with the lowest bridge identifier becomes
the designated bridge. Since the root path cost of the root bridge is
zero, the root bridge automatically becomes the designated bridge for
the segments connected onto it.
♦ Bridge Identifier This is a combination of the bridge priority (a
parameter you can set) and the bridge MAC address (a unique,
unchangeable number set at the factory). A lower bridge identifier
results to a higher priority for the bridge, thus increasing its chance of
being selected as the root bridge.
♦ Root Path Cost The root path cost of a bridge is the sum of the path
cost of the port from which a packet is forwarded and the root path
costs of all the bridges the packet goes through. The root path cost of
the root bridge is zero.
♦ Bridge Priority This parameter can be set. The smaller the number
you set, the higher the bridge priority is. The higher the bridge
priority, the more chance the bridge has of becoming the root bridge.