Moxa EDS-82810G [44/113] How stp works

EDS-828 Series Featured Functions

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What happens if a link failure is detected? As shown in next figure, the STP process reconfigures the network

so that traffic from LAN segment 2 flows through Bridge B.

STP will determine 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 sections below.

STP Requirements

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

• Communication 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.

• Each 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-828 is 32768.

• Each port has a cost that specifies the efficiency of each link. The efficiency 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: