3Com corebuilder 3500 Implementation Manual page 529

Roving Analysis 529
The RMON groups that require samples of traffic from the ASICs will
not work because they do not receive any traffic data when a port is
defined as a monitor port. The system is capable of doing either roving
analysis or traffic sampling, but not both at the same time.
The monitor and analyzer ports must be of the same type media. For
example, FDDI ports cannot be monitored on Ethernet ports and
Ethernet ports cannot be monitored on FDDI ports.
You can use a Fast Ethernet port to monitor a Gigabit Ethernet port,
but a warning message will be printed. Because the analyzer port is
slower than the monitor port, the analyzer port may not see all
frames.