Siemens SIMATIC NET SCALANCE M826 Operating Instructions Manual page 51

DP
RP
AP
To allow data traffic via the ring, enable the "Spanning Tree" function in the WBM in "Layer 2
> Spanning Tree".
Spanning tree prevents loops being formed in the network by allowing only one path and
disabling the other (redundant) SHDSL interfaces for data traffic. If there is an interruption,
the data can be sent over an alternative path. The functionality of the spanning tree
algorithm is based on the exchange of configuration and topology change frames.
The role of every SHDSL device in the ring is detected automatically either as a root bridge or
bridge. There is only ever one root bridge in the ring.
The parameters for Rapid Spanning Tree are preset in the factory and cannot be changed:
• Hello Time : 2 seconds
• Forward Delay Time : 15 seconds
• Maximum Aging Time : 20 seconds
• Switch Priority : 32768
• Port Priority: 128
Since all SHDSL devices use the same defaults, the SHDSL device with the lowest MAC
address becomes the root bridge.
The best loop-free path in the ring is decided by specifying the role of the ports in the ring.
The ports with the lowest path costs to the root bridge are root ports.
The ports leading away from the root bridge are designated ports.
The alternate port shows the alternative path to the root bridge.
The root port and the designated port forward traffic, the alternate port, on the other hand,
does not allow any packets through to prevent a loop in the ring.
The root port receives a hello frame (60 bytes) at intervals of 2 seconds. If this frame is not
received three times in a row, the ring topology is automatically recalculated.
Path Cost
SCALANCE M826
Operating Instructions, 10/2022, C79000-G8976-C362-07
Designated Port
Root Port
Alternate Port
Connecting up
5.5 SHDSL connection
51