Frequently Asked Questions on STE10/100A

Does the STE10/100A support multicast address filtering? How can we setup multicast address table on STE10/100A?
The STE10/100A does indeed support multicast address filtering. It does so through a hashing mechanism like that used by the 2114x family of devices. Setting bit 7 in CSR6 register allows it to receive incoming frames with a multicast destination address (first bit in the destination address field is 1). Incoming frames with multicast destination addresses are subjected to imperfect filtering. Frames with physical destination addresses are checked against the single physical address in CSR25 & CSR 26. CSR26 and CSR27 are used to contain the 64-bit hash table used for imperfect multicast address filtering. Bit 10 of Receive Descriptor RDES0 is used to indicate that the received frame has a multicast address.

For any incoming frame with a multicast destination address, the STE10/100A applies the standard Ethernet cyclic redundancy check (CRC) function and then it uses the most significant 6 bits of the result as a bit index into the table. If the indexed bit is set, the frame is accepted. If the bit is cleared, the frame is rejected. Below is sample C code of a hash index for a given Ethernet address. This filtering mode is called imperfect because multicast frames not addressed to this particular STE10/100A may be received, but it does significantly decreases the number of unwanted frames will be received.

Little Endian Architecture Hash C Routine:

#define CRC32_POLY 0xEDB88320UL /* CRC-32 Poly -- Little Endian*/
#define HASH_BITS 9                     /* Number of bits in hash */ unsigned crc32_mchash(unsigned char *mca)
{
u_int idx, bit, data, crc = 0xFFFFFFFFUL; for (idx = 0; idx < 6;
idx++)
for (data = *mca++, bit = 0; bit < 8; bit++, data >>=1)
crc = (crc >> 1) ^ (((crc ^ data) & 1) ? CRC32_POLY : 0);
return crc & ((1 << HASH_BITS) - 1) /* return low bits for hash */
}