Spanning Tree Protocol
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 24 and 25 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 25 on Switch 2, which cuts off communication between the
switches for VLAN 2.
Two VLANs on one instance of Spanning Tree Protocol
In the following figure, VLAN 1 and VLAN 2 belong to different Spanning Tree Groups. The two instances
of spanning tree separate the topology without forming a loop, so that both VLANs can forward packets
between the switches without losing connectivity.
Two VLANs on separate instances of Spanning Tree Protocol