Spanning Tree Protocol

The relationship between ports, trunk groups, VLANs, and spanning trees is shown in the following table. Table 11 Ports, trunk groups, and VLANs

Switch elementBelongs to

 

 

Port

Trunk group, or one or more VLANs

 

 

Trunk group

Only one VLAN

 

 

VLAN (non-default)

One Spanning Tree Group

 

 

Assigning cost to ports and trunk groups

When you configure a trunk group to participate in a Spanning Tree Group, all ports must have the same Spanning Tree configuration, as follows:

•port priority

•path cost

•link type

•Edge port status

•Port Fast Forward status

Assign lower path costs on each member of a trunk group, to ensure the trunk group remains in the Forwarding state.

Multiple Spanning Trees

Each switch 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 independent links on two switches 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 supports IEEE 802.1d Spanning Tree Protocol, and may contain more than one VLAN. All other STGs support Per VLAN Spanning Tree (PVST+), and may contain only one VLAN each. The switch can support multiple VLANs in STGs 2-128; however, you must enable IEEE 802.1s Multiple Spanning Tree Protocol mode. For more information, see the “RSTP and MSTP” chapter in this guide.

Why do we need Multiple Spanning Trees?

The following figure shows a simple example of why we need multiple Spanning Trees. This example assumes that port 20 and 21 are not part of a Trunk Group. Two VLANs (VLAN 1 and VLAN 2) exist between Switch 1 and Switch 2. If the same Spanning Tree Group is enabled on both switches, the switches see an apparent loop and block port 21 on Switch 2, which cuts off communication between the switches for VLAN 2.

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