currently identified iobs have at most 28 ports on the ds 1 iob therefore spid values of 28 30 are unused but could be used in the future all remaining spid values will be configured based on the type of iob or ncb frames with invalid spid values will be counted and discarded size 1 bit usage the discard priority dp 312 field is used in congestion situations where it indicates the discard priority for the frame a value of 1 one indicates that the frame is discard priority a value of 0 zero indicates that the frame is not discard priority in a congestion situation discard priority frames will be discarded in preference to non discard priority frames at an ingress port any discard priority indication included in the incoming datagrams will be copied into the dp bit 312 at an egress port the dp bit 312 will be copied into the appropriate datagram field the dp bit 312 can be set by the datagram originator by preceding network elements or anywhere within the ocular switching equipment where policing is being performed size 1 bit usage forward tag congestion notification or u201c forward tagging u201d ftag 315 is set to 1 one by the ocular switching equipment to indicate that congestion is being experienced for traffic in the direction of the fairfax frame 201 carrying the ftag 315 indication thus ftag 315 indicates to the frame destination that congestion was experienced along the frame 39 s path through the ocular switching equipment at an ingress port any congestion indications included in incoming datagrams are carried transparently through the ocular switching equipment the congestion values are not copied to the fairfax header ftag field 315 at an egress port if the fairfax header ftag field 315 is set to 1.