D-Link DGS-3308FG User Manual page 230

8-port Gigabit Ethernet Switch User's Guide
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.
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