RackSwitch G8000 Application Guide
Chapter 5: Spanning Tree
75BMD00041, November 2008
Bridge Protocol Data Units (BPDUs)
To create a Spanning Tree, the switch generates a configuration Bridge Protocol Data Unit
(BPDU), which it then forwards out of its ports. All switches in the Layer 2 network participat-
ing in the Spanning Tree gather information about other switches in the network through an
exchange of BPDUs.
A BPDU is a 64-byte packet that is sent out at a configurable interval, which is typically set for
two seconds. The BPDU is used to establish a path, much like a “hello” packet in IP routing.
BPDUs contain information about the transmitting bridge and its ports, including bridge MAC
address, bridge priority, port priority, and path cost.
The generic action of a switch upon receiving a BPDU is to compare the received BPDU to its
own BPDU that it will transmit. If the received BPDU is better than its own BPDU, it will
replace its BPDU with the received BPDU. Then, the switch uses this information to block any
necessary ports.
Determining the Path for Forwarding BPDUs
When determining which port to use for forwarding and which port to block, the G8000 uses
information in the BPDU, including each bridge 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 spanning-tree bridge priority:
RS G8000 (config)# spanning-tree stp 1 bridge
priority <0-61440, in steps of 4096>
