Allied Telesis AlliedWare Plus GS980MX Series Installation Manual page 69

In stacks of three or four 28- or 52-port switches the amount of inter-switch
network traffic might require a stacking trunk with greater bandwidth than
that provided by the default ports of:
GS980MX/28PSm and GS980MX/28 ports 27 and 28.

GS980MX/52PSm and GS980MX/52 ports 51 and 52.

For such situations these additional ports can be used:
GS980MX/28PSm and GS980MX/28 ports 25 and 26.

GS980MX/52PSm and GS980MX/52 ports 49 and 50.

GS980MX Series Installation Guide for Virtual Chassis Stacking
69