AL103 Revision 1.0
3.2 Frame Forwarding
After a frame is received, both source address (SA) and destination address (DA) are retrieved. The
SA is used to update the port’s address table and the DA is used to determine the frames destination
port.
The Address Lookup Engine will attempt to match the destination address with the addresses stored
in the address table. If there is a match found, a link between the source port and the destination
port is then established.
If the first bit of the destination address is “0,” the frame is regarded as an unicast frame. The
destination address is passed to the Address Lookup Engine, which returns a matched destination
port number to identify which port the frame should be forwarded to. If the destination port is
within the same VLAN of the receiving port, the frame will be forwarded. If the destination port
does not belong to the VLANs specified at the receiving ports, the frame will be discarded. The
event will be recorded as a VLAN boundary violation.
There are two ways that the AL103 handles frames with unknown destinations. The forwarding
decision is controlled by the Flood Control option (System Configuration Register 00). If Flood
Control is disabled, the frame will be forwarded to all ports (except the receiving port) within the
same VLANs of the receiving port. If the Flood Control option is enabled, the AL103 will forward
the frame only to the uplink port specified at the receiving port.
Note: The AL103 defines a port as either a single port or a trunk.
If the port monitoring function is enabled, the frame forwarding decision is also subject to the port
monitoring configurations.
If the first bit of the destination address is a “1,” the frame will be handled as a multicast or
broadcast frame. The AL103 does not differentiate multicast frames from broadcast frames except
for the reserved bridge management group address, as specified in table 3.5 of IEEE 802.1d
standard. The destination ports of the broadcast frame are all ports within the same VLAN except
the source port itself.
3.2.1 Broadcast Storm Control
One of the unique features provided by the AL103 is Broadcast Storm Control. This option allows
the user to limit the number of broadcast frames into the switch. This option can be implemented on
a per port basis. A threshold number of broadcast frames can be programmed in System Register II
(register 01).
When Storm Control is enabled and the number of cumulated non-unicast frames is over the
programmed threshold, the broadcast frame is discarded.
If Storm Control is disabled, or the number of non-unicast frames received is not over the
programmed threshold, the AL103 will forward the frame to all ports (except the receiving port)
specified within the VLANs at the receiving port.
If Broadcast-Storm-drop (BConly_SC) is enabled in System Register III (register 02), the AL103
will only drop broadcast frames but not the multicast frames.
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Reference Only / Allayer Communications
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