D-Link DES-3326 User Manual page 410

24-port NWay Ethernet Switch User's Guide
This is a common network design. The switches C and D h ave
redundant links to the backbone switches A and B using trunks.
Trunks, by default, carry all the VLAN traffic from VLAN 1 and
VLAN 2. So switch C is not only receiving traffic for VLAN 1,
but it is also receiving unnecessary broadcast and multicast
traffic for VLAN 2. It is also blocking one port for VLAN 2.
Thus, there are three redundant paths between switches A and B
394 Understanding and Troubleshooting the Spanning Tree Protocol