Technical data

BLADE OS 5.1 Application Guide

 Chapter 5: Spanning Tree Group BMD00136, November 2009

The relationship between port, trunk groups, VLANs, and Spanning Trees is shown in Table 9.

Note – Due to Spanning Tree’s sequence of listening, learning, and forwarding or blocking,

lengthy delays may occur. You can use Port Fast Forwarding to permit a port that participates in

Spanning Tree to bypass the Listening and Learning states and enter directly into the Forwarding

state. While in the Forwarding state, the port listens to the BPDUs to learn if there is a loop and, if

dictated by normal STG behavior (following priorities, and so on), the port transitions into the

Blocking state. This feature permits the G8000 to interoperate well within Rapid Spanning Tree

networks.

Bridge Protocol Data Units (BPDUs)

BDPU Overview

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 participating 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 necessary ports.

Table 9 Ports, Trunk Groups, and VLANs

Switch Element Belongs To

Port Trunk group

One or more VLANs

Trunk group One or more VLANs

VLAN (non-default) One Spanning Tree Group