EDS-408A/405A Series User’s Manual

Featured Functions

LAN 1

Bridge B

Bridge A

LAN 2

Bridge C

LAN 3

STP determines which path between each bridged segment is most efficient, and then assigns a specific reference point on the network. When the most efficient path has been identified, the other paths are blocked. In the previous 3 figures, STP first determined that the path through Bridge C was the most efficient, and as a result, blocked the path through Bridge B. After the failure of Bridge C, STP re-evaluated the situation and opened the path through Bridge B.

How STP Works

When enabled, STP determines the most appropriate path for traffic through a network. The way it does this is outlined in the following sections.

STP Requirements

Before STP can configure the network, the system must satisfy the following requirements:

yCommunication between all the bridges. This communication is carried out using Bridge Protocol Data Units (BPDUs), which are transmitted in packets with a known multicast address.

yEach bridge must have a Bridge Identifier that specifies which bridge acts as the central reference point, or Root Bridge, for the STP system—bridges with a lower Bridge Identifier are more likely to be designated as the Root Bridge. The Bridge Identifier is calculated using the MAC address of the bridge and a priority defined for the bridge. The default priority of EDS is 32768.

yEach port has a cost that specifies the efficiency of each link. The efficieny cost is usually determined by the bandwidth of the link, with less efficient links assigned a higher cost. The following table shows the default port costs for a switch:

Port SpeedPath Cost 802.1D,Path Cost

 

1998 Edition802.1w-2001

10 Mbps

100

2,000,000

100 Mbps

19

200,000

1000 Mbps

4

20,000

STP Calculation

The first step of the STP process is to perform calculations. During this stage, each bridge on the network transmits BPDUs. The following items will be calculated:

yThe bridge that should be the Root Bridge. The Root Bridge is the central reference point from which the network is configured.

yThe Root Path Costs for each bridge. This is the cost of the paths from each bridge to the Root Bridge.

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