Token Ring Error Msg: Bad FC/AR flags--Probable Causes

Several conditions can cause a token ring Sniffer to report that a frame has "Bad FC/AR Flags." This article discusses the most probable ones.

THE FRAME STATUS (FS) FIELD

A token ring frame contains several fields used to control the frame itself. The last of these fields--the end of frame sequence (EFS)--contains two bytes, the second of which--the frame status (FS)--is the most probable source of the error message. The FS looks like this: where: A = 1 if the destination address was recognized
A = 0 if the destination address was not recognized
C = 1 if frame was copied (accepted at least at the MAC level for processing)
C = 0 if frame was not copied

TWO POSSIBLE FS FIELD ERROR CONDITIONS

ONE: The 1s and 0s don't match. For example, if the FS is: it is considered in error, because the first A (1) says the destination address is recognized, and the second (0) says it is not. The C's also do not match.

TWO: Some of the 0's are 1's. For example, if the FS is: the third bit is a 1 and it should be a zero.

If your Sniffer trace can display the FS, check it for either of these conditions. If you find one of them, the most probable cause is hardware, especially if only one machine is returning the error message. Hardware does not necessarily mean the network interface card (NIC), but the card is a good place to start. If it is connected through UTP, check the connection between the card's DB-9 and the UTP cable. This device sometimes causes errors, although when it does you normally see network problems in other parts of the frame.

NON-DATA SYMBOLS

Token rings use Manchester encoding, and you may be getting a J or a K (non-data symbols) in the FS.

MAXTU MISMATCH

If you are running across bridges, you might have a MAXTU mismatch between your machines and bridges.

JITTER

Finally, you could be running into a token ring timing phenomenon called jitter, although it normally shows up in numerous frames, not just one. Around 1990, a TI chipset popular with OEMs for token ring cards was prone to jitter. The problems were resolved but they surface from time to time. Check the hardware.