Nortel Web OS 10.0, Application Manual

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Web OS 10.0 Application Guide
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Chapter 2: VLANs
212777-A, February 2002
Bridge Protocol Data Units (BPDUs)
To create a Spanning Tree, the Web switch generates a configuration Bridge Protocol Data
Unit (BPDU), which it then forwards out of its ports. All switches in the Layer 2 network par-
ticipating 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 Web switch adds its own bridge ID num-
ber and increments the path cost of the BPDU. The Web 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, Web switches use
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.
For more information on bridge priority, port priority, and port cost, refer to the Web OS 10.0
Command Reference. Much like least-cost routing, root cost assigns lower values to high-
bandwidth ports, such as Gigabit Ethernet, to encourage their use. For example, a 10-Mbps
link has a “cost” of 100, a 100-Mbps (Fast Ethernet) link carries a cost of 19, and a 1000-Mbps
(or Gigabit Ethernet) link has a cost of 4. The objective is to use the fastest links so that the
route with the lowest cost is chosen.