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Alteon OS Application Guide
Chapter 5: Spanning Tree Group
10742C4911, January 2007
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 and
MAC addresses, bridge priority, port priority, and path cost. If the ports are tagged, each port
sends out a special BPDU containing the tagged information.
The generic action of a switch on 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 adds its own bridge ID number
and increments the path cost of the BPDU. The switch uses this information to block any nec-
essary ports.
Determining the Path for Forwarding BPDUs
When determining which port to use for forwarding and which port to block, the GbE Switch
Module 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 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:
/cfg/l2/stg x/brg/prio
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 con-
figure the port priority:
/cfg/l2/stg x/port x/prio