DM8108
8 port 10/100M Fast Ethernet Switching Controller
Preliminary
21
Version: DM8108-DS-P02
November 25, 1999
The receive port utilizes the RDVCLK to clock in the
received data into FIFO and uses RXTOG requests for a
Receiving Buffer block. The switching engine will execute
the similar process as for the Ethernet Ports.
The transmit port appends the Sync. field to a normal
Ethernet packet and sends the packet out through TD(3:0)
at the rising edge of TXENCLK.
TXENCLK
TD(3:0)
Nib n-1
NIb n
Nib n+1
Switching Engine
All the packet switching is processed by the Switching
Engine, which has following functions:
MAC Address Learning Process
The DM8108 has a self-learning mechanism for learning
the MAC addresses of attached Fast Ethernet devices in
real time. The DM8108 searches for the source address of
an incoming packet in the Address Table and acts as
follows:
If the source address was not found in the Address Table,
the DM8108 waits until the end of the packet (no error) and
updates the Address Table.
If the source address was found in the Address Table, the
DM8108 waits for a good packet received indication.
Address Recognition
The DM8108 forwards the incoming packets to appropriate
port(s) according to the Destination Address as follows:
T
If the packet is from a local port--
1)
If it is a Unicast address and the address is found
in the Address Table, the DM8108 will:
. If the port number recorded is matched to port
number on which the packet received, the packet
is discarded.
. If the port numbers are different, the packet is
forwarded to the appropriate port.
2)
If it is a Unicast address and the address is not
found in the Address Table, the DM8108 acts as if
the packet is a Multicast packet and forwards it to
the Expansion Transmit port and all the local
ports except the incoming port,
3)
If it is a Multicast/Broadcast address, the packet is
forwarded to the Expansion Transmit port and all
local ports (except to the port on which the packet
was received).
T If the packet is from Expansion Bus—
1)
If it is a Unicast address specified in the
Destination Address in the Ethernet Packet, the
DM8108 will:
. If the recorded port pointed to a local port, the
packet will be forwarded to that port.
. If the destination address is not found (not
recorded by the Mac address learning process),
the packet will be forwarded to all the local ports
and the Expansion Transmit port.
2)
If it is a Multicast/Broadcast address (destination
device # should set invalid), the packet will be
forwarded to all the local ports and the Expansion
Transmit port.
Address Aging
The DM8108 includes hardware to support for automatic
address aging.
Buffers and Queues
The DM8108 incorporates 3 transmit queues and one
common receive buffer area for the two Fast Ethernet ports
and the Expansion Port, The queues and buffers are
located in the DRAM along with Address Table. The
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