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Smart Managed Compact Switch
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PHOENIX CONTACT 8089_en_03
Components of a Spanning Tree domain
Designated switch
The switch that connects a specific LAN segment (with the lowest path costs) to the root
switch.
Root port
The other switches set the port with the lowest path costs (or with the highest total transmis-
sion speed) as the root switch in the forwarding state.
There is always just one root port per switch.
Exception: The switch supports several Spanning Tree domains.
Designated ports
Ports in the forwarding state of the designated switch.
These are the ports with the “best” path to the root switch.
Switch ID
Priority and
MAC address
The switch with the lowest bridge identifier is the root switch. The bridge identifier consists
of the MAC address and the priority. Since the priority is placed before the MAC address,
assignment of the appropriate priority clearly identifies the root switch, independent of the
MAC address. The switch with the highest priority (lowest value) becomes the root switch.
For every switch port within the network, a unique cost calculation is created. These root
path costs are the sum of all path costs for one packet on the path between the root switch
and corresponding switch port. The port of a switch with the lowest root path costs is always
the active port. If the same root path costs have been calculated for two or more ports, the
switch priority followed by the port priority determine the priority of the path.
Port ID
The port identifier consists of the path costs and the priority. Since the priority is placed be-
fore the path costs, the assignment of the appropriate priority clearly identifies the root port,
independent of the path costs. The port with the highest priority (lowest value) becomes the
root port.
5.2.5.5 Processes in the Spanning Tree Protocol (STP)
Selecting the root switch
For every topology modification, every switch first assumes that it is the root switch and thus
sends its own switch ID (e.g., the MAC address) into the network. All switches receive these
messages (MAC multicast) and store the contents of the “best” message. The “best” mes-
sage contains the following topology information: The root ID information and the cost infor-
mation.
After having received the root ID information, the switch compares the following:
–
The new root ID is saved if it has a higher priority than the IDs that are already stored
(including its own ID).
– The path costs are checked in case the root ID is identical to the one already saved. If
they are lower, the ID is saved.
– If the root ID and the costs are the same, the ID of the sender is checked. If the ID is
lower than the switch's own ID, it is saved.