User Documentation

User Manual Managed Switches
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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.
3.5.6.2 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 Weidmüller 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. The
following table shows the default port costs for a switch:
Port Speed
Path Cost 802.1D,
1998 Edition
Path Cost
802.1w-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:
 Which bridge should be the Root Bridge. The Root Bridge is the central reference point from
which the network is configured.
 The Root Path Costs for each bridge. This is the cost of the paths from each bridge to the Root
Bridge.
 The identity of each bridge’s Root Port. The Root Port is the port on the bridge that connects to
the Root Bridge via the most efficient path. In other words, the port connected to the Root Bridge
via the path with the lowest Root Path Cost. The Root Bridge, however, does not have a Root
Port.
 The identity of the Designated Bridge for each LAN segment. The Designated Bridge is the
bridge with the lowest Root Path Cost from that segment. If several bridges have the same Root
Path Cost, the one with the lowest Bridge Identifier becomes the Designated Bridge. Traffic