Determining the path for forwarding BPDUs
When determining which port to use for forwarding and which port to block, the switch uses information
in the BPDU, including each bridge priority ID. A technique based on the lowest root cost is then
computed to determine the most efficient path for forwarding.
The bridge priority parameter controls which bridge on the network is the STP root bridge. To make one
switch the root bridge, configure the bridge priority lower than all other switches and bridges on your
network. The lower the value, the higher the bridge priority. The bridge priority is configured using the
The port priority helps determine which bridge port becomes the designated port. In a network topology
that has multiple bridge ports connected to a single segment, the port with the lowest port priority
becomes the designated port for the segment. The port priority is configured using the following
/cfg/l2/stp y/port x/prior
Port path cost
The port path cost assigns lower values to high-bandwidth ports, such as Gigabit Ethernet, to encourage
their use. The objective is to use the fastest links so that the route with the lowest cost is chosen. A value
of zero indicates that port cost is computed dynamically based on link speed. This works when forcing
link speed, so it does not just apply to auto negotiated link speed.
By default, all switch ports have the path cost set to 2. To use dynamic path cost, based on link speed, set
the path cost to 0 (zero). For example, if the path cost is set to zero:
A 10 Gbps link receives a path cost of 2
A 100 Mbps link receives a path cost of 19
Configure the port path cost using the following command:
/cfg/l2/stp y/port x/cost
Spanning Tree Group configuration guidelines
This section provides important information on configuring Spanning Tree Groups (STGs).
Default Spanning Tree configuration
In the default configuration, a single STG with the ID of 1 includes all ports on the switch. It is called the
default STG. All other STGs (except the default STG) are empty, and VLANs must be added by the user.
You cannot assign ports directly to an STG. Add the ports to a VLAN, and add the VLAN to the STG.
STGs 1-127 are enabled by default and assigned an ID number from 1 to 127. STG 128 is disabled by
default, and contains the management VLAN 4095.
An STG cannot be deleted, only disabled. If you disable the STG while it still contains VLAN members,
Spanning Tree will be off on all ports belonging to that VLAN.
Spanning Tree Protocol