KSZ9563RNXC-TR datasheet(35/226 Pages) MICROCHIP

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DS00002419D-page 35

KSZ9563R

4.4.5

TRAFFIC CONDITIONING & POLICING

4.4.5.1

Weighted Random Early Detection (WRED)

WRED is an optional feature that 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.

When buffer utilization reaches a maximum threshold, the probability has reached 1 and all incoming packets are

dropped.

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.

WRED statistically drops more packets from large users than small. Therefore, traffic sources that generate the most

traffic are more likely to be slowed down than traffic sources that generate little traffic.

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

Only packets to and from the

processor are forwarded.

Learning is disabled.

transmit enable = 0

receive enable = 0

learning disable = 1

The processor should program the “Static MAC

Table” with the entries that it needs to receive (for

example, BPDU packets). The “overriding” bit should

be set so that the switch will forward those specific

packets to the processor. The processor may send

packets to the port(s) in this state. Address learning

is disabled on the port in this state.

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