TNETX4090
ThunderSWITCH II 9-PORT 100-/1000-MBIT/S ETHERNET SWITCH
SPWS044E ± DECEMBER 1997 ± REVISED AUGUST 1999
frame format on the NM port (continued)
To provide a CRC word, which includes the header, the NM port generates a new CRC word as the frame is being read out. It simultaneously checks the existing CRC in the frame and, if an error is found, ensures that the final byte of the newly generated CRC is corrupted to contain an error, too. The CRC word is deliberately corrupted if the header parity protection (described in the following) indicates an error in the header. In either case, the pfe bit also is set to 1 after the final byte of the frame has been read from NMData.
If the frame was received on a port other than the NM port, then the crctype bit is set according to whether an IEEE Std 802.1Q tag header was inserted into the frame during ingress.
±If crctype = 1, a header was inserted.
±If crctype = 0, a header was not inserted (crctype also is 0 if the frame VLAN ID was 0x000 and was replaced by the port VLANID (PVID) from the PortxQTag register).
In an IEEE Std
±All other bits in the byte are reserved and are 0. The second TPID byte output contains:
D
DBit 5 protects the first byte of the TPID field (i.e., the one containing crctype and source port number).
DBit 6 protects the first byte of the VLAN ID field.
DBit 7 protects the second byte of the VLAN ID field.
DAll other bits in the byte are reserved and are 0.
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| TPID (Tag Protocol Identifier) |
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| TCI (Tag Control Information) |
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| 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| Priority |
| cfi |
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| VLAN ID |
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| 5 4 3 | 2 1 | 0 7 6 | 5 4 3 | 2 1 | 0 |
| 7 | 6 | 5 | 4 | 3 | 2 1 | 0 7 6 | 5 4 |
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7 |
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| 0 |
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Destination |
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| Source |
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| 802.1Q header |
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| Data |
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| FCS |
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Address |
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| Address |
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| TPID |
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| TCI |
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6 Bytes |
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| 6 Bytes |
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| 2 Bytes |
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| 46±1517 Bytes |
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| 4 Bytes | |||||||||||||||||||||||||||||
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| 2 Bytes |
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| Byte 1 |
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| Byte 2 |
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| Odd Parity Bits |
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| CRC |
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| Reserved |
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| Source |
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| 2nd |
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| Reserved |
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| Type |
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| Port |
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| TCI |
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| TCI |
| TPID |
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| Byte |
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| Byte |
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7 |
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| 6 |
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| 0 |
| 7 |
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| 3 | 2 | 1 | 0 |
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Figure 2. NM Frame Format
POST OFFICE BOX 655303 •DALLAS, TEXAS 75265 | 29 |
