TNETX3190 datasheet(26/66 Pages) TI1

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TNETX3190

ThunderSWITCH

™ 16/3 ETHERNET™ SWITCH

WITH 16 10-MBIT/S PORTS AND 3 10-/100-MBIT/S PORTS

SPWS046B – FEBRUARY 1998 – REVISED APRIL 1999

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

port-routing-code pretag on reception

If the pretag bit is set to 1 in the appropriate Portxcontrol register, during the seven MxxRCLK cycles prior to

MxxRXDV going high, the port expects to receive a seven-nibble pretag on MxxRXD3–MxxRXD0 (see

Figure 4).

MxxRCLK

MxxRXD3–

MxxRXD0

Preamble and Frame

Tag 6

MxxRXDV

Tag 5

Tag 4

Tag 3

Tag 2

Tag 1

Tag 0

Figure 4. Port-Routing-Code Pretag

Each of the 28 bits contained within these nibbles represents a destination port for the frame. If a bit is 1, the

frame is queued to that port. If the port is disabled or its link is inactive, the frame subsequently is drained from

the port’s queue, which again returns to zero length.

The port assignments for these tag bits are shown in Table 6.

Table 6. Received Pretag Port Assignments

TAG

MxxRXD3

MxxRXD2

MxxRXD1

MxxRXD0

Port 27 (NM)

Port 26

Port 25

Port 24

Port 23

Port 22

Port 21

Port 20

Port 19

Port 18

Port 17

Port 16

Port 15

Port 14

Port 13

Port 12

Port 11

Port 10

Port 09

Port 08

Port 07

Port 06

Port 05

Port 04

Port 08

Port 02

Port 01

Port 00

The 28 bits are examined during the reception process to see if just one destination bit is set. If this is the case,

the frame is received and handled like a unicast frame (such frames can be cut through). If more than one bit

is set, the frame is handled as an in-order-broadcast (and cannot be cut through). The frame is routed to all the

port(s) specified, regardless of whether the destination address is unicast or multicast (i.e., the destination

address is not examined).

If all 28 tag bits are 0, the frame is discarded. If the frame has not been discarded in the MAC (for some reason),

the portx-filtered RX-frames statistic is incremented.

The tag bits are not examined to see if the source port is specified as a destination port, so it is possible, for

example, for port 25 to send a frame to itself by setting bit 1 in tag 6.

The IALE sees and processes pretagged frames exactly as nonpretagged frames (it does not know that a frame

has been pretagged). However, the final port-routing code generated by the IALE is ignored (the information

in the pretag determines the destination ports). Normal IALE behavior occurs in terms of address learning and

interrupt generation and statistics updates, with one exception – the portx-filtered RX-frames statistic is

incremented if the pretag contained all 0s. Whether or not the IALE generates its own (ignored) port-routing code

of all 0s has no effect on this statistic if the frame is a pretagged frame.

Since the IALE’s routing decision is ignored on pretagged frames, the Txblockports, Rxuniblockports, and

Rxmultiblockports registers have no effect on frame reception or transmission.

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