NXP Semiconductors PXN2020, Reference Manual, Page: 603 / 1376

Fast Ethernet Controller (FEC)
PXN20 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor 25-37
Figure 25-27. Ethernet Address Recognition—Microcode Decisions
25.4.9 Hash Algorithm
The hash table algorithm used in the group and individual hash filtering operates as follows. The 48-bit
destination address is mapped into one of 64 bits, which are represented by 64 bits stored in GAUR, GALR
(group address hash match) or IAUR, IALR (individual address hash match). This mapping is performed
by passing the 48-bit address through the on-chip 32-bit CRC generator and selecting the 6 most
significant bits of the CRC-encoded result to generate a number between 0 and 63. The MSB of the CRC
result selects GAUR (MSB = 1) or GALR (MSB = 0). The least significant 5 bits of the hash 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.
For example, 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 desired addresses.
The effectiveness of the hash table declines as the number of addresses increases.
Receive Address
I/G Address
?
Exact Match
?
Hash Search
Group Table
Match
?
Hash Search
Individual Table
False
Match
?
False
False
True
True
True
NOTES:
FCE - field in RCR register (Flow Control Enable)
I/G - Individual/Group bit in Destination Address (least significant bit in first byte received in MAC frame)
Individual
Group
True
False
True
False
?
Pause Address
FCE
?
Recognition
Reject Frame
Flush from FIFO
Reject Frame
Flush from FIFO
Receive Frame
Receive Frame
Receive Frame
Receive Frame