Texas Instruments TNETX3270 specifications Received Pretag Port Assignments, Tag, Port 27 NM

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

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±	Tag 6	Tag 5	Tag 4	Tag 3	Tag 2	Tag 1	Tag 0	Preamble and Frame
MxxRXD0

MxxRXDV

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
6	Port 27 (NM)	Port 26	Port 25	Port 24
5	Port 23	Port 22	Port 21	Port 20
4	Port 19	Port 18	Port 17	Port 16
3	Port 15	Port 14	Port 13	Port 12
2	Port 11	Port 10	Port 09	Port 08
1	Port 07	Port 06	Port 05	Port 04
0	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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