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FEC Frame Transmission
Receive Frame
Receive Frame
Figure 11-4. Ethernet Address Recognition Flowchart
11.4.4 Hash Table Algorithm
The hash table process used in the group hash filtering operates as follows. When a frame
with the destination address I/G bit set is received by the FEC, the 48-bit destination media
access control (MAC) address is mapped into one of 64 bins, which are represented by 64
bits stored in HASH_TABLE_LOW, _HIGH. This is performed by passing the 48-bit MAC
address through the on-chip 32-bit CRC generator and selecting 6 bits of the CRC-encoded
result to generate a number between 0 and 63. Bit 31 of the CRC result selects
HASH_TABLE_HIGH (bit 31 = 1) or HASH_TABLE_LOW (bit 31 = 0). Bits 30–26 of
the CRC result select the bit within the selected register. If the CRC generator selects a bit
that is set in the hash table, the frame is accepted; otherwise, it is rejected. The result is that
if eight group addresses are stored in the hash table and random group addresses are
received, the hash table prevents roughly 56/64 (or 87.5%) of the group address frames
from reaching memory. Those that do reach memory must be further filtered by the
processor to determine if they truly contain one of the eight preferred addresses.
11-8
Check Address
I/G Address
?
G
Broadcast
True
Address
?
False
True
Hash Match
?
False
False
Promiscuous
Mode
?
False
(R_CNTRL[PROM] = 0)
Discard Frame
MCF5272 User's Manual
I
True
Perfect Match
?
True (R_CNTRL[PROM] = 1)
Receive Frame
Receive Frame
Set Miss Bit
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