3Com 5500-SI Configuration Manual page 87

Multiple Fabric Port Candidates 87
A port cannot be a fabric port if the jumboframe function is enabled on the port. So
make sure the jumboframe function is disabled on a port if you want to configure the
port to be a fabric port.
With a port group of a switch being the current fabric port group, you need to
invalidate the current fabric port group before configuring the other port group to be
a fabric port group.
After a fabric is configured, the master switch synchronizes its configuration file to all
the units in the fabric. As the Flashes of the units may differ in size, the synchronizing
operation may fail on certain units because of lack of Flash memory space, which
makes the fabric fails to be established. So make sure each unit has enough free Flash
memory space before configuring a fabric.