zl50407 Zarlink Semiconductor, zl50407 Datasheet - Page 39

• Manufacturer Part Number: zl50407
• Description: Lightly Managed/unmanaged 8-port 10/100m + 1-port 10/100/1000m Ethernet Switch
• Manufacturer: Zarlink Semiconductor

7.6.1
As already discussed, the WRED mechanism may drop frames on output queue status. In addition to these
reasons for dropping, we also drop frames when global buffer space becomes scarce. The function of buffer
management is to make sure that such dropping causes as little blocking as possible. If a received frame is
dispatched to the best effort queue, the buffer management will check on the overall buffer situation plus the output
queue status to decide the frame drop condition. If the source port has not enough buffer for it, the frame will be
dropped. If the output queue reach the UCC (unicast congest control) and the shared buffer has run out, the frame
will be dropped by b%. If the output queue reach the UCC and the source port reservation is lower than the buffer
low threshold, the frame will be dropped. All the dropping functions are disabled if the source port is flow control
capable.
7.7
Because frame loss is unacceptable for some applications, the ZL50407 provides a flow control option. When flow
control is enabled, scarcity of source port buffer space may trigger a flow control signal; this signal tells a source
port sending a packet to this switch, to temporarily hold off.
While flow control offers the clear benefit of no packet loss, it also introduces a problem for quality of service. When
a source port receives an Ethernet flow control signal, all microflows originating at that port, well-behaved or not,
are halted. A single packet destined for a congested output can block other packets destined for un-congested
outputs. The resulting head-of-line blocking phenomenon means that quality of service cannot be assured with high
confidence when flow control is enabled.
On the other hand, the ZL50407 will still prioritize the received frame disregarding the outgoing port flow control
capability. If a frame is classified as high priority, it is still subjected to the WRED, which means the no-loss on the
high priority queue is not guaranteed. To resolve this situation, the user may set the output port WRED threshold so
high that may never be reached, or program the priority mapping table in the queue manager to map all the traffic to
best effort queue on the flow control capable port. The first method has side impact on the global resource
management since the port may hold too much per class resource that is scarce in the system. The second
method, by nature, lost the benefit of prioritization.
See Programming Flow Control Registers application note, ZLAN-44, for more information.
7.7.1
For unicast frames, flow control is triggered by source port resource availability. Recall that the ZL50407’s buffer
management scheme allocates a reserved number of FDB slots for each source port. If a programmed number of a
source port’s reserved FDB slots have been used, then flow control Xoff is triggered.
Xon is triggered when a port is currently being flow controlled, and all of that port’s reserved FDB slots have been
released.
Note that the ZL50407’s per-source-port FDB reservations assure that a source port that sends a single frame to a
congested destination will not be flow controlled.
7.7.2
Flow control for multicast frames is triggered by a global buffer counter. When the system exceeds a programmable
threshold of multicast packets, Xoff is triggered. Xon is triggered when the system returns below this threshold.
Note:
If per-port flow control is on, QoS performance will be affected.
Flow Control Basics
Dropping When Buffers Are Scarce
Unicast Flow Control
Multicast Flow Control
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ZL50407
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Data Sheet