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TNETX4090
ThunderSWITCH II
9-PORT 100-/1000-MBIT/S ETHERNET
SWITCH
SPWS044E – DECEMBER 1997 – REVISED AUGUST 1999
29
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
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 802.1D-compliant application, the header simply can be removed from the frame to produce
a headerless frame with a correct CRC word.
– All other bits in the byte are reserved and are 0.
The second TPID byte output contains:
Odd-parity protection bits for the other three bytes in the tag header
Bit 5 protects the first byte of the TPID field (i.e., the one containing crctype and source port number).
Bit 6 protects the first byte of the VLAN ID field.
Bit 7 protects the second byte of the VLAN ID field.
All other bits in the byte are reserved and are 0.
TPID (Tag Protocol Identifier) TCI (Tag Control Information)
Destination
Address
Source
Address
TPID TCI
Length/Type
FCS
(CRC-32)
Data
Reserved
75436
2nd
TCI
Byte
Odd Parity Bits
Reserved
Source
Port
Byte 1 Byte 2
CRC
Type
802.1Q header
1st
TCI
Byte
1st
TPID
Byte
2107 5436210
10000 0001 0000000
Priority cfi VLAN ID
6 Bytes
6 Bytes
2 Bytes
2 Bytes 46–1517 Bytes2 Bytes
4 Bytes
7
6543 21076543210 7 6 5 4 32107654 3 2 1 0
Figure 2. NM Frame Format