3Com 1000 - Manual (Page 129)

Using the Switch C-5
To avoid this situation, we recommend that you
connect the two SuperStack II Switch units using a
Virtual LAN Trunk (VLT). For more information
about VLTs, refer to
“Connecting Common VLANs
Between Switch Units” on page 5-3 .
Figure C-1 Network configuration that results in loss of contact