Why Do We Need Multiple Spanning Trees; Switch-Centric Spanning Tree Group; Figure 5-1:Using Multiple Instances Of Spanning Tree Group - Nortel Alteon OS 42C4911 Application Manual

Why Do We Need Multiple Spanning Trees?

Figure 5-1
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.
Figure 5-1 Using Multiple Instances of Spanning Tree Group

Switch-Centric Spanning Tree Group

In Figure 5-2 on page
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
shows a simple example of why we need multiple Spanning Trees. Two VLANs,
112, VLAN 2 is shared by application switch A and GbE Switch Module
BladeCenter
GbE Switch GbE Switch
Module B Module B
Chapter 5: Spanning Tree Group
Alteon OS Application Guide
111