RackSwitch G8000 Application Guide
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Chapter 5: Spanning Tree BMD00041, November 2008
Why do we need multiple Spanning Trees?
The following examples describe why we need multiple spanning trees.
In Figure 5-1, VLAN 1 and VLAN 2 pass traffic between switch 1 and switch 2. If you have a
single Spanning Tree Group, the switches see an apparent physical loop, and one VLAN may
become blocked, affecting connectivity, even though no logical loop exists. VLAN 2 traffic is
blocked unnecessarily.
Figure 5-1 Two VLANs on one Spanning Tree Group
In Figure 5-2, VLAN 1 and VLAN 2 belong to different Spanning Tree Groups. 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 5-2 Two VLANs, each on a different Spanning Tree Group
Switch 1
VLAN 1, STG 1
Switch 2
VLAN 2, STG 1
X
VLAN 2 traffic blocked by STG 1
Switch 1
VLAN 1, STG 1
Switch 2
VLAN 2, STG 2
VLAN 1 passes traffic on STG 1
VLAN 2 passes traffic on STG 2
