Allied Telesis AT-S105 - Manual (Page 154)

Chapter 13: Spanning Tree and Rapid Spanning Tree Protocols
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You can avoid this problem by connecting the switches using tagged
instead of untagged ports when you plan to have STP or RSTP enabled
on your network. If each port connecting the two bridges is a tagged
member of all three VLANs, then traffic for each of the VLANs can still flow
through one the data links if the other two are blocked by Spanning Tree.
The second and third data links act as redundant links in case the primary,
unblocked data link becomes disabled. See Figure 48 for an example of
this solution.
Figure 48. STP and VLAN Compatibility with Tagged Ports
Note
For information on tagged and untagged ports, refer to Chapter 3,
“VLAN Overview” on page 46.
Blocked (Redundant) Data Links
Ports Blocked by STP
T T T
T T T
VLANs VLANs
1 - 3 1 - 3 1 - 3
VLANs VLANs VLANs
1 - 3 1 - 3 1 - 3
VLANs VLANs VLANs