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CrossFire 8600/8605 Token-Ring Switches v. 1.2, P/N: 710001641 Switch Theory of Operation
•
Block destination address at a specific port—this prevents the specified port
from sending frames to a specified destination.
•
Allow destination address at specific ports—this indicates that the specified
port must send frames to the specified destinations
only
.
•
Force destination address to a specific port—this allows forwarding to a
unicast address that has not been learned. It can also be used to limit the
forwarding of Multicast addresses to a subset of ports. This last filter applies
to non-source-routing frames
only
.
Congestion Control
At regular intervals, the switch CPU inspects the queues on all output ports. If a
queue size is above a certain threshold, the port is instructed to:
•
Set the transmit priority for low priority frames to a specified high level
•
Delete old frames from the queue until it reaches a specified size
When the queue size again comes below a normal threshold size the port is
instructed to set the transmit priority back to the normal level.
Three Switching Modes
Cut-Through
In this mode the switch starts forwarding the packet to the output port as soon as
the destination address or the source-route of the incoming packet has been
resolved. This technique ensures very low latency, typically in the range of 30-100
µs. However, if errors occur on the input port during the reception of a packet, the
error will still be forwarded to the output port. Note that cut-through can only be
used in transmissions between ports which operate at 16 Mbps.
Store and Forward
In this mode, the switch receives the total packet from the input port, checks it for
any errors and then starts forwarding the packet to the destination port. This
technique will ensure that no faulty packets are transmitted by output port. The
negative impact however, is higher latency, typically in the range of 40–2,000 µs
depending on the packet size. Though slower than cut-through mode, this is still
much faster that conventional bridges.
Auto (Adaptive Cut-Through)
This is a technique whereby the switch will automatically swap between store-and-
forward and cut-through modes based on an error threshold. If the number of
received faulty packets is low, then cut-through mode is used; if the number of