TNETX3270 datasheet(21/66 Pages) TI1

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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

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

frame format on the NM port (continued)

TPID (Tag Protocol Identifier)

TCI (Tag Control Information)

Destination

Address

Source

Address

TPID

TCI

Length/Type

FCS

(CRC-32)

Data

Reserved

2nd

TCI

Byte

Odd Parity Bits

Reserved

Source

Port

Byte 1

Byte 2

CRC

Type

802.1Q header

1st

TCI

Byte

1st

TPID

Byte

10 0 0 0

0 00 0 0 00 Priority cfi

VLAN ID

6 Bytes

2 Bytes

46–1517 Bytes

2 Bytes

4 Bytes

7 6 5 4 3 2 1 0 7 6 5 4 3 21 0

7 6

4 3 21 07 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:

D crc = 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).

D crc = 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.

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