Virtual Local Area Network (VLAN) isolation disrupts packet forwarding. The following figure illustrates the problem. Two VLANs (V1 and V2) connect four devices, and both VLANs are in the same spanning tree group. V2 includes three of the four devices, whereas V1 includes all four devices. After a spanning tree protocol detects a loop, it blocks the link with the highest link cost. In this case, the 100 Mbps link is blocked, which isolates a device in V2. To avoid this problem, either configure V2 on all four devices or use MSTP with a different Multiple Spanning Tree Instance (MSTI) for each VLAN.
