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.
3.6.4.2 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
Bridge B
Bridge C
L
A
N
1
L
A
N
2
L
A
N
3
Bridge A
