KSZ9567RTXI-TR datasheet(42/232 Pages) MICROCHIP

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KSZ9567R

DS00002329D-page 42

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4.4.14.2

Time Aware Traffic Scheduler (TAS)

Like the Credit-Based Shaper, the Time Aware Scheduler is used to regulate the egress flow of high priority traffic. The

TAS defines a periodic window of time during which only scheduled (high priority) traffic may egress a port. Additionally,

transmission of low priority traffic will not be initiated during a guard band period prior to the high priority transmit window.

This ensures that no blocking can occur during the transmit window.

The TAS is configured individually for each egress port. Typically the egress port shall be configured for multiple queues

- either 2 or 4 - with at least one queue reserved for unscheduled “best effort” traffic. For each port, only one queue may

be set up for TAS. With a 4-queue configuration, it is possible to have both TAS queue and one or two Credit-Based

Shaper queues at the same time.

Once the guard band period begins, no packet transmission will be initiated unless the non-TAS queue packet length is

smaller than the remaining guard band time. Any packets already being transmitted will be allowed to complete trans-

mission. The guard band period shall be set up to match the transmit time for the largest possible non-scheduled packet.

This ensures that transmission is always complete before the end of the guard band time. The guard band time is imme-

diately followed by the transmit window. During the transmit window, transmission is allowed only for the TAS queue. All

other queues remain blocked. At the end of the transmit window, the block is lifted from all queues, and they are allowed

to transmit per the scheduling scheme - either strict priority or weighted round robin (WRR). The cycle then repeats with

start of the next guard band period.

A 128-entry table is used to define the time intervals that relate the guard band, transmit window and cycle time. The

size of the table makes it possible to define two or more concurrent cycles which correspond to two or more individual

streams. Each cycle can have a unique period, guard band and transmit window time.

4.4.15

LOW LATENCY CUT-THROUGH MODE

Cut-Through Mode allows for the reduction of packet latency through the switch by forwarding packets directly to the

egress port without first waiting for receipt of the entire packet. This feature works in conjunction with the Time Aware

Traffic Scheduler (TAS), and applies only if the incoming packet is being sent to a destination port with a corresponding

TAS queue. Cut-through mode is enabled or disabled individually for each egress port. The TAS must be enabled when

using this mode.

Cut-through mode does not guarantee that all packets in the scheduled traffic class can be expedited. The following

conditions must be met in order for cut-through to occur for a given packet. If the conditions are not met, the packet will

be stored and queued in the usual manner.

• The Time Aware Scheduler must be applied to a queue at the egress port.

• The packet must belong to the scheduled traffic class that uses the Time Aware Scheduler.

• The TAS transmission window must be open.

• No other packet is currently being transmitted at that port.

• The TAS queue for that port is empty.

• The speed of the egress port must be equal to the speed of the ingress port.

• The packet must be unicast.

The only traffic going to a cut-through egress port should be a single unicast cut-through stream. The egress port may

be configured as a single queue, with the Time Aware Scheduler applied to that one queue. Set the transmit window to

100%.

When cut-through occurs, the packet is forwarded after the first 64 bytes have been received. Latency through the

switch is approximately 900ns for 1Gb/s traffic, regardless of the packet size. For non-cut-through packets, the minimum

latency is proportional to the size of the packet.

4.4.16

INGRESS MAC ADDRESS FILTERING FUNCTION

When a packet is received, the destination MAC address is looked up in both the static and dynamic MAC address

tables. If the address is not found in either of these tables, then the destination MAC address is “unknown”. By default,

an unknown packet is forwarded to all ports except the port at which it was received. An optional feature makes it pos-

sible to specify the port or ports to which to forward unknown packets. It is also possible to specify no ports, meaning

that unknown packets will be discarded. This feature is implemented separately for unknown unicast, unknown multicast

and unknown VID packets.

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