Stack Trunk; Figure 11: Stack Of Two Switches - Allied Telesis AT-x550-18XTQ Installation Manual

Chapter 2: Virtual Chassis Stacking

Stack Trunk

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The two switches of a stack are connected together with a trunk of QSFP+
transceivers installed in slots 17 and 21. The transceivers must be from
Allied Telesis and be approved for use as a stack trunk for x550 Series
switches. The following transceivers were approved at the time this
manual was written:
 AT-QSFPSR4 transceiver - requires 12-strand OM4 fiber optic
cable and has a maximum operating distance of 150m (492 ft).

AT-QSFPLR4 transceiver - requires single-mode fiber optic cable
and has an operating range of 2m (6.6 ft) to 10km (6.2 mi).
 AT-QSFP1CU and AT-QSFP3CU direct connect cables in lengths
of 1 and 3m, respectively
The connections of the QSFP+ transceivers in slots 17 and 21 in the two
switches of a stack must cross over. The transceiver in slot 17 in one
switch has to connect to the transceiver in slot 21 in the other switch. An
example is illustrated in Figure 11.
Figure 11. Stack of Two Switches
Caution
The switches will not form the stack if the connections to the QSFP+
transceivers in slots 17 and 21 do not cross over. They will instead
operate as stand-alone devices.  E78
Note
Transceivers must be purchased separately.