Displays the current PIPE setting for a port, and allows
it to be changed. Pipes "tunnel" frames from one port to
another. Exactly which frames are piped is controlled by
PIPEFLAG.
If two numeric arguments are supplied, a "frame pipe" is set
up from <port> to [destport], otherwise the current setting
for the specified port is displayed.
If the second argument is zero, piping is disabled.
Modified settings remain in force until changed or system is
restarted.