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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
26
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 FrameTag 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
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.