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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
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 RequirementsBefore 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
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 Speed | Path Cost 802.1D, | Path Cost |
| 1998 Edition | |
10 Mbps | 100 | 2,000,000 |
100 Mbps | 19 | 200,000 |
1000 Mbps | 4 | 20,000 |
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.