Westermo RedFox Series - Manual (Page 351)

Westermo OS Management Guide
Version 4.17.0-0
16.1.2 Bridge Identity
Each bridge is assigned an 8 byte bridge identifier (bridge ID) as shown in fig. 16.2 .
4 bits 12 bits 6 bytes
System ID
Extension
System ID
Unique Bridge Address (MAC)
Priority
Figure 16.2: Structure of bridge ID.
The bridge ID is divided into a priority part (4 bits) and a system ID (60 bits). The
bridge with the lowest bridge ID within the LAN will become the root bridge, i.e.,
lower priority means greater chance to become root bridge. The bridge ID is also
used to select a designated bridge on a link, when multiple bridges on the link
have the same ”least cost path” to the root bridge.
The format of the bridge ID follows IEEE std. 802.1D-2004 (RSTP). It differs from
the structure specified in IEEE std. 802.1D-1998 (STP), where the priority field
was 2 bytes and the system ID field was 6 bytes. The change in structure was
made with respect to the multiple spanning tree protocol (MSTP) defined in IEEE
std. 802.1Q-2005 (WeOS currently does not support MSTP).
❼ Priority (4 bits): Can take values in range 0-15, where 8 is default. 0 (zero)
means highest priority and 15 lowest priority. Compared to the ”old” 2 byte
priority field of STP, this is rather a priority factor field, which can be multi-
plied by 4096 to get the ”old” STP priority.
❼ System ID Extension (12 bits): Set to all zeros in WeOS.
❼ Unique Bridge Address: Tie-breaker ensuring the bridge ID will be unique.
WeOS uses the base MAC address assigned to the switch for this field.
16.1.3 Path Cost
Each port is associated with a cost referred to as a path cost. Low-speed links are
generally given a high cost, which increases the probability of the port ending up
in blocking state (and vice versa), in case spanning tree discovers a loop.
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