Moxa IKS-6728A-4GTXSFP-24-24-T [22/37] How stp works

Managed Ethernet Switch Redundancy Protocol (UI 2.0) STP/RSTP/MSTP

5-3

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 assign 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:

• All bridges must be able to communicate with each other. The 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. For example, the default priority setting of Moxa switches 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.

Bridge B

Bridge C

LAN 1

LAN 2