The optional Length argument specifies the length of the test
    data part of each loop message. The maximum and minimum
    permitted values depend on the particular data link in use.
    The range is 0-4294967295.  The default length is 40.

    loop  [node node-id] mop circuit circuit-name -
          address lan-address, -
          length integer [,optional arguments...]

    If an Address is not supplied in the command on a LAN circuit,
    then the Client argument must be specified so the address
    information can be obtained from the MOP Client database.

    loop  [node node-id] mop circuit circuit-name -
          client client-name, -
          length integer [,optional arguments...]

    Examples:

    loop mop circuit csmacd-1 address 08-00-2b-00-13-4F, length 20

    loop mop circuit fddi-1 -
         client mynode, length 45  ! Client gives default Address