KSZ9567RTXI datasheet(36/232 Pages) MICROCHIP

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KSZ9567R

DS00002329D-page 36

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4.4.5

TRAFFIC CONDITIONING & POLICING

4.4.5.1

Two Rate Three Color Marker

The Two Rate Three Color Marker meters an IP packet stream and marks its packets green, yellow, or red. A packet is

marked red if it exceeds the Peak Information Rate (PIR). Otherwise, it is marked either yellow or green depending on

whether it exceeds or doesn't exceed the Committed Information Rate (CIR).

The Meter operates in one of two modes. In the Color-Blind mode, the Meter assumes that the packet stream is uncol-

ored. In the Color-Aware mode, the Meter assumes that some preceding entity has pre-colored the incoming packet

stream so that each packet is green, yellow, or red. The Marker (re)colors an IP packet according to the results of the

Meter.

4.4.5.2

Weighted Random Early Detection (WRED)

The WRED feature monitors the average queue size of packet memory and ingress queue size of each traffic class,

and drops packets based on memory and queue utilization. If the buffers are almost empty, all incoming traffic is

accepted. As the buffer utilization increases, the probability for dropping an incoming packet also increases.

WRED is intended to avoid the problem of global synchronization. Global synchronization can occur when a switch

becomes congested and begins dropping incoming packets all at once. For TCP streams, packet drops invoke the TCP

congestion control mechanism, which reduce the transmission rate until there are no more packet drops. If there are

many TCP streams and their congestion control mechanisms act in unison, this can cause an undesirable oscillation in

traffic rates. By selectively dropping some packets early rather than waiting until the buffer is full, WRED avoids dropping

large numbers of packets at once and minimizes the chances of global synchronization.

The packet drop probability is based on the minimum threshold, maximum threshold, and a probability multiplier. When

the average queue depth is above the minimum threshold, packets start getting dropped. The rate of packet drop

increases linearly as the average queue size increases until the average queue size reaches the maximum threshold.

The probability multiplier is the fraction of packets dropped when the average queue depth is at the maximum threshold.

When the average queue size is above the maximum threshold, all packets are dropped.

AVB traffic streams (SR streams) can be exempted from WRED policing.

4.4.6

SPANNING TREE SUPPORT

To support spanning tree, one port is the designated port for the host processor, which is defined as the port for which

tail tagging is enabled. Each of the other ports can be configured in one of the five spanning tree states via “transmit

enable”, “receive enable” and “learning disable” register bits. Table 4-12 shows the setting and software actions taken

for each of the five spanning tree states.

TABLE 4-12:

SPANNING TREE STATES

Disable State

Port Setting

Software Action

The port should not forward

or receive any packets.

Learning is disabled.

transmit enable = 0

receive enable = 0

learning disable = 1

The processor should not send any packets to the

port. The switch may still send specific packets to the

processor (packets that match some entries in the

“Static MAC Table” with “overriding bit” set) and the

processor should discard those packets. Address

learning is disabled on the port in this state.

Blocking State

Port Setting

Software Action

Only packets to the proces-

sor are forwarded.

Learning is disabled.

transmit enable = 0

receive enable = 0

learning disable = 1

The processor should not send any packets to the

port(s) in this state. The processor should program

the “Static MAC Table” with the entries that it needs

to receive (for example, BPDU packets). The “over-

riding” bit should also be set so that the switch will

forward those specific packets to the processor.

Address learning is disabled on the port in this state.

Listening State

Port Setting

Software Action

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