Technical data

BLADE OS 5.1 Application Guide
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 Chapter 5: Spanning Tree Group BMD00136, November 2009
Multiple Spanning Trees
Each RackSwitch G8000 supports a maximum of 128 Spanning Tree Groups (STGs). Multiple
STGs provide multiple data paths, which can be used for load-balancing and redundancy.
You enable load balancing between two G8000s using multiple STGs by configuring each path with
a different VLAN and then assigning each VLAN to a separate STG. Each STG is independent.
Each STG sends its own Bridge Protocol Data Units (BPDUs), and each STG must be
independently configured.
The STG, or bridge group, forms a loop-free topology that includes one or more virtual LANs
(VLANs). The switch supports 128 STGs running simultaneously. The default STG 1 may contain
an unlimited number of VLANs. All other STGs 2-128 may contain only one VLAN each.
Default Spanning Tree Configuration
In the default configuration, a single STG with the ID of 1 includes all ports on the switch. It is
called the default STG. Although ports can be added to or deleted from the default STG, the default
STG (STG 1) itself cannot be deleted from the system.
All other STGs are empty and VLANs must be added by the user. However, you cannot assign ports
directly to an STG. Instead, add ports to a VLAN first and then add the VLAN to the STG.
Each STG is enabled by default, and assigned an ID number from 2 to 128.
Why Do We Need Multiple Spanning Trees?
The following examples describe why we need multiple spanning trees.
In Figure 9, 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.