Nortel Networks 42C4911 manual Why Do We Need Multiple Spanning Trees?

Alteon OS Application Guide

Why Do We Need Multiple Spanning Trees?

Figure 5-1shows a simple example of why we need multiple Spanning Trees. Two VLANs, VLAN 1 and VLAN 100 exist between application switch A and GbE Switch Module B. If you have a single Spanning Tree Group, the switches see an apparent loop, and one VLAN may become blocked, affecting connectivity, even though no actual loop exists.

If VLAN 1 and VLAN 100 belong to different Spanning Tree Groups, then the two instances of Spanning Tree separate the topology without forming a loop. Both VLANs can forward packets between the switches without losing connectivity.

BladeCenter

GbE Switch

Module B

Figure 5-1Using Multiple Instances of Spanning Tree Group

Switch-Centric Spanning Tree Group

In Figure 5-2 on page 112, VLAN 2 is shared by application switch A and GbE Switch Module B on ports 8 and 17 respectively. Application Switch A identifies VLAN 2 in Spanning Tree Group 2 and GbE Switch Module B identifies VLAN 2 in Spanning Tree Group 1. Spanning Tree Group is switch-centric—it is used to identify the VLANs participating in the Spanning Tree Groups. The Spanning Tree Group ID is not transmitted in the BPDU. Each Spanning Tree decision is based on the configuration of that switch.

42C4911, January 2007 Chapter 5: Spanning Tree Group „ 111