3Com 5500-EI PWR Reference Manual page 954

For each port, only one RMON statistics entry can be created. That is, if an RMON statistics entry was
already created for a given port, you will fail to create a statistics entry with a different index for the port.
You can use the display rmon statistics command to display the information about the statistics entry.
Examples
# Add the statistics entry numbered 20 to take statistics of Ethernet 1/0/1.
<Sysname> system-view
System View: return to User View with Ctrl+Z.
[Sysname] interface Ethernet 1/0/1
[Sysname-Ethernet1/0/1] rmon statistics 20
2-15