Allied Telesis AT-GS950/24 - Manual (Page 121)

AlliedWare Plus Version 2.1.2 Management Software Web Browser User’s Guide
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Figure 38. 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 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 39 for an example of
this solution.