IP178C
Preliminary Data Sheet
43/91
October 27, 2005
Copyright © 2004, IC Plus Corp.
IP178C-DS-R09
Special tag
IP178C supports special tag function to exchange switching information with CPU without involving VLAN
tag information. The special tag function is enabled by programming MII register 31.30[14] STAG_EN.
From CPU to switch
When special tag function is enabled, IP178C forwards packets from MII (CPU) by checking special tag
added by CPU. The tag definition is shown in the following table. IP178C will remove the special tag
81XX and re-calculate CRC when it forwards the packet to a un-tag field. IP178C will update the special
tag to 81XX and re-calculate CRC when it forwards the packet to a tag field.
Preamble
SFD
DA
SA
81XX(special tag)
Data
CRC
Special tag 81XX
bit [15:12]
bit[11:8]
bit[7:0]
8
1
0000_0001: instruct 178C forwards the packet to port 0
0000_0010: instruct 178C forwards the packet to port 1
0000_0100: instruct 178C forwards the packet to port 2
0000_1000: instruct 178C forwards the packet to port 3
0001_0000: instruct 178C forwards the packet to port 4
0010_0000: instruct 178C forwards the packet to port 5
0100_0000: instruct 178C forwards the packet to port 6
1000_0000: instruct 178C forwards the packet to port 7
From switch to CPU
When special tag function is enabled, IP178C sends packets to MII (CPU) with source port information
by adding special tag to the frame. IP178C will add the special tag 81XX and re-calculate CRC when it
receives the packet from a un-tag field. IP178C will update the tag 8100 to 81XX and re-calculate CRC
when it receives the packet from a tag field. The tag definition is shown in the following table.
Bit[15:12]
bit[11:8]
bit[7:0]
8
1
0000 0001: the source port of the packet is port 0
0000 0010: the source port of the packet is port 1
0000 0100: the source port of the packet is port 2
0000 1000: the source port of the packet is port 3
0001 0000: the source port of the packet is port 4
0010 0000: the source port of the packet is port 5
0100 0000: the source port of the packet is port 6
1000 0000: the source port of the packet is port 7
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