Here's a short tutorial on how to implement a default behavior for your BFM using VMM.
Some protocols require constant activity on their interface even when you don't have any data to transmit. This means you must have a mechanism that drives idle packets or dummy data items as long as the generator doesn't produce items for its BFM. In VMM, Generators are connected to BFMs using vmm_channels and we''re just about to show you how to take advantage of that for our needs.

Note: Basic familiarity with SystemVerilog and VMM is assumed.

task bfm_main();
 forever begin
 // NON-DEFAULT BEHAVIOR: check if there''s anything waiting in the channel
 if (chan.size() > 0) begin 
 this.notify.reset(DONE);
 chan.get(packet);
 
 drive_packet(packet);
 
 // DEFAULT BFM BEHAVIOR: if channel is empty, drive an idle packet
 end else begin
 drive_idle_packet();
 end
 // and.. don''t forget to provide stopping condition
 wait_if_stopped(); 
 end
 endtask