TNETX4080GGP datasheet(25/64 Pages) TI1

25 / 64 page

TNETX4080

ThunderSWITCH II™ 8-PORT 10-/100-MBIT/S ETHERNET™ SWITCH

SPWS050A – AUGUST 1998 – REVISED NOVEMBER 1998

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

frame format on the NM port (continued)

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 is providing 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 mimics the presence of IEEE Std 802.1Q TPID of 81–00 (ethertype

constant) 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 being 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 are required to 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 check 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.

When a frame is transmitted on the NM port, no header stripping occurs (again, because the NM port does not

have a PortxQTag register or txacc bit), so the frame read by the host software contains one header (or possibly

more, depending on how the frame was received).

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 and sets the reserved bits to 0. Frames received from the host via the NM port are required

to contain a valid IEEE Std 802.1Q VLAN ID (VID) in the third and fourth bytes following the source address.

(The NM port does not have a default VLAN ID register for inserting a VLAN tag if none is provided. It cannot

also be configured as an access port.) Frames that do not contain a valid tag are incorrectly routed. They also

can be corrupted at the transmission port(s) as the tag-stripping process does not check that the four bytes after

the source address are a valid tag because they are valid tags under all other circumstances.

When a frame is transmitted on (read from) the NM port, no tag stripping occurs (because the NM port does

not have the default VLAN ID register or access configuration control), so the frame read by the host software

can contain one or more header tags, depending on how the frame was received.

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