Technical data

BLADE OS 5.1 Application Guide
BMD00136, November 2009 Chapter 5: Spanning Tree Group
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Determining the Path for Forwarding BPDUs
When determining which port to use for forwarding and which port to block, the RackSwitch
G8000 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.
Bridge Priority
The bridge priority parameter controls which bridge on the network is the STG root bridge. To
make one switch become 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. Use the following
command to configure the bridge priority (Global Configuration mode):
Port Priority
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. Use the following command to configure the
port priority (Interface Port mode):
Port Path Cost
The port path cost assigns lower values to high-bandwidth ports, such as Gigabit Ethernet, to
encourage their use. The cost of a port also depends on whether the port operates at full-duplex
(lower cost) or half-duplex (higher cost). For example, if a 100-Mbps (Fast Ethernet) link has a
“cost” of 10 in half-duplex mode, it will have a cost of 5 in full-duplex mode. The objective is to use
the fastest links so that the route with the lowest cost is chosen. A value of 0 (the default) indicates
that the default cost will be computed for an auto-negotiated link or trunk speed.
>> # spanning-tree stp <x> bridge priority
>> # spanning-tree stp
<x> priority