Phoenix Contact 2700290, User Manual, Page: 103 / 154

(Rapid) Spanning Tree
8089_en_03 PHOENIX CONTACT
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– If the root ID, costs, and sender ID are the same, the priority of the sender port is the
decisive criterion.
Selecting a designated switch
For every network, the switch with the most favorable root connection is selected. This
switch is called the designated switch.
The root switch is the designated switch for all directly connected networks.
Selecting a root port
Once the root switch has been specified by processing the root IDs, the switches now spec-
ify the root ports.
The most favorable path is specified by minimizing all connection costs on the path to the
root switch. Transmission speeds can also serve as costs. For the switch, the path costs
added by each port for every HOP (the hop of a data packet from one point to the next) are
preset to a value of 19 (default setting/recommended for 100 Mbps) and can be modified at
any time by the user.
Selecting a designated port
At every “designated switch” the port with the most cost-effective data link in the direction of
the root switch is called the designated port.
Port costs
The port costs can be set according to two different standards, 802.1D (STP) or 801.1W
(RSTP).
If, in addition to Phoenix Contact devices, devices from other manufacturers are used, it
is recommended that the port costs are set according to a uniform standard.
The “dot1dstpPathCostDefault” SNMP object (OID 1.3.6.1.2.1.17.2.18) can be used to
change the standard that is used.
Table 5-2 Port costs according to 802.D
Transmission speed Recommended value Recommended range
10 Mbps 100 50 - 600
100 Mbps 19 10 - 60
Table 5-3 Port costs according to 802.W
Transmission speed Recommended value Recommended range
10 Mbps 2 000 000 200 000 - 20 000 000
100 Mbps 200 000 20 000 - 2 000 000
1000 Mbps 20 000 2 000 - 200 000