Allied Telesis AT-GS950/24 - Manual (Page 79)

AT-GS950/24 Web Interface User Guide
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Figure 26. STP and VLAN Fragmentation with Untagged Ports
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 of 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 27 on page 80
for an example of this solution.