D-Link DES-3326 User Manual - page 391
Manual is about: 24-Port Fast Ethernet Plus 2-Port Gigabit Module Layer 3 Switch
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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
Understanding and Troubleshooting the Spanning Tree Protocol
377
This is a common network design. The switches C and D have
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 and two blocked
ports per VLAN. This increases the chance of a data loop.