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IDT Ethernet Interfaces
Address Recognition Logic
79RC32438 User Reference Manual
11 - 8
November 4, 2002
Notes
There are two types of destination addresses, individual addresses and group addresses. An individual
address is associated with a particular station on the network, while a group address is associated with one
or more stations on the network. A group address can be further classified as either a multicast address (an
address associated by a higher level convention with a group of logically related stations) or a broadcast
address (an address that denotes the set of all stations on a given LAN).
The Ethernet interface supports up to four station addresses. A station address is a 48-bit MAC address
stored in a station address low and high register pair. There are four station address register pairs:
ETH[0|1]SAL[0|1|2|3]
ETH[0|1]SAH[0|1|2|3]
These are programmed with the four individual station addresses of the Ethernet interface. The destina-
tion address of an Ethernet packet is compared to all four station addresses. If the destination address of
the packet matches any one of the four station addresses, the Ethernet packet is accepted.
Applications that do not require all four station addresses may program the same address into multiple
station address register pairs. For example, if an application requires the Ethernet interface to have only a
single station address, all four station address register pairs can be assigned the same station address
value. The MAC address used for control frames is contained in the ETH[0|1]CFSA[0|1|2] registers. Using
another example, if an application requires two addresses, one address would be assigned to station one
and the other address would be assigned to stations two, three, and four.
A hash table approach is used to determine if multicast group destination address packets should be
accepted.
1
When a packet with a multicast group destination address is received, a 6-bit hash value is
computed by passing the 48-bit destination address through the frame check sequence CRC calculator.
The hash value, consisting of bits 26 through 31 of the computed CRC, is used as an index into a 64 bin
hash table in which each bin is represented by a single bit. If the selected bit in the hash table is a one and
the Accept Filtered Multicast Packets (AFM) bit in the Ethernet address recognition control (ETH[0|1]ARC)
register is set, the packet is accepted.
The 64-bit hash table is stored in the HASH[0|1] registers. HASH0 contains bits 0 through 31 of the hash
table, while HASH1 contains bits 32 through 63 of the hash table.
The hash table filtering algorithm is not perfect, and therefore packets must be further filtered by soft-
ware to determine if they do, in fact, match a multicast address that should be accepted. If the Accept All
Multicast Packets (AM) bit in the ETH[0|1]ARC register is set, all multicast packets are accepted regardless
of whether or not they pass the hash table filtering algorithm.
A broadcast address is a MAC address consisting of all ones (that is, FF-FF-FF-FF-FF-FF). If the
Accept Broadcast Packets (AB) bit in the ETH[0|1]ARC register is set, all broadcast packets are accepted
by the Ethernet interface. When this bit is cleared, all broadcast packets are rejected. When a packet is
accepted by the Ethernet interface, three bits are updated in the DEVCS field of a DMA descriptor.
The Filter Match (FM) bit is set when one of the following conditions occurs:
–
The packet matches an individual station address
–
The packet passes the hash table filtering algorithm described above
–
The packet is a multicast packet and was accepted because the AM bit in the ETH[0|1]ARC
register was set
–
The packet is a broadcast packet and was accepted because the AB bit in the ETH[0|1]ARC
register was set.
The Multicast Packet (MP) bit is set when the accepted packet is a multicast packet, and the Broadcast
Packet (BP) bit is set when the accepted packet is a broadcast packet.
The Ethernet interface has a promiscuous mode which is enabled by setting the Promiscuous Mode
(PRO) bit in the ETH[0|1]ARC register. In this mode, the address recognition logic accepts all incoming
packets regardless of their destination address. While in this mode, the Filter Match (FM) bit in the DEVCS
field of a DMA descriptor is still set only in the conditions outlined above. The address filtering algorithm is
summarized in Figure 11.6.
1.
The only exception to this is the multicast address 01-80-c2-00-00-01 which is always received regardless of the
setting of the corresponding Ethernet hash table entry.