Texas Instruments TNETX3270 specifications Vlan ID, Fcs, Tpid TCI, Crc

TNETX3270

ThunderSWITCH 24/3 ETHERNET SWITCH

WITH 24 10-MBIT/S PORTS AND 3 10-/100-MBIT/S PORTS

SPWS043B ± NOVEMBER 1997 ± REVISED APRIL 1999

frame format on the NM port (continued)

						TPID (Tag Protocol Identifier)																				TCI (Tag Control Information)
		1		0			0	0	0	0	0		1	0	0	0		0		0	0	0	0	Priority				cfi					VLAN ID
						5 4 3				2 1		0 7 6					5 4 3				2 1		0	7	6	5	4		3	2 1		0 7 6		5 4
		7
			6																																	3 2 1		0
Destination								Source						802.1Q header										Length/Type									Data						FCS
Address								Address						TPID						TCI																		(CRC-32)
6 Bytes								6 Bytes												2 Bytes					2 Bytes					46±1517 Bytes								4 Bytes
													2 Bytes
													Byte 1																						Byte 2
																												Odd Parity Bits
	CRC				Reserved												Source										2nd			1st			1st			Reserved
	Type																		Port								TCI			TCI			TPID
																											Byte			Byte			Byte
7					6			5				4			3			2					1			0	7			6		5		4		3	2	1		0

Figure 2. NM Frame Format

Any device reading frames out of the NM port must expect frames to be in the format shown in Figure 2.

Frames received into the switch on the NM port also must conform to this format, with the following caveats:

Dcrc = 0 in NMRxcontrol

When the host provides a frame containing valid CRC it also must provide in the TPID field valid header parity protection and indicate via the crctype bit which type of CRC the frame contains [i.e., including the header (crctype = 0), or excluding the header (crctype = 1)]. If crctype indicates that the header is included, as for NM port transmissions, this pretends that IEEE Std 802.1Q TPID of 81±00 (ethertype constant) is present in the TPID field. If a CRC error or parity error is detected, the frame is discarded.

When crctype indicates that the header is included, the NM port regenerates CRC to exclude the header during the reception process (this converts the frame into the required internal frame format).

Dcrc = 1 in NMRxcontrol

If the switch is asked to generate a CRC word for the frame, the values in the TPID field are ignored by the NM port. The switch inserts header parity protection. It replaces the final four bytes of the frame with the calculated CRC (the values in the final four bytes provided are don't care).

In either case, the NM port inserts its own port number into the source port field in the least significant bits of the first TPID byte, sets the crctype bit to 0, and also sets the reserved bits to 0.

Frames received from the host via the NM port must contain a valid IEEE Std 802.1Q VLAN ID in the third and fourth bytes, following the source address (the NM port does not have a PortxQtag register for inserting a VLAN tag if none is provided and does not have an rxacc bit). Frames that do not contain a VLAN tag are incorrectly routed. They also can be corrupted at the transmission port(s). The header-stripping process does not verify that the two bytes after the source address are a valid IEEE Std 802.1Q TPID because there is a valid header under all other circumstances.

POST OFFICE BOX 655303 •DALLAS, TEXAS 75265 21