
MVTX2803
Data Sheet
24
Zarlink Semiconductor Inc.
Figure 4 - Buffer Partition Scheme Used in the MVTX2803AG
7.8.1 Dropping When Buffers Are Scarce
Summarizing the two examples of local dropping discussed earlier in this chapter:
If a queue is a delay-bounded queue, we have a multilevel WRED drop scheme, designed to control delay
and partition bandwidth in case of congestion.
If a queue is a WFQ-scheduled queue, we have a multilevel WRED drop scheme, designed to prevent
congestion.
In addition to these reasons for dropping, the MVTX2803AG also drops frames when global buffer space
becomes scarce. The function of buffer management is to ensure that such droppings cause as little blocking as
possible.
7.9 MVTX2803AG Flow Control Basics
Because frame loss is unacceptable for some applications, the MVTX2803AG provides a flow control option.
When flow control is enabled, scarcity of buffer space in the switch 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
uncongested outputs. The resulting head-of-line blocking phenomenon means that quality of service cannot be
assured with high confidence when flow control is enabled.
In the MVTX2803AG, each source port can independently have flow control enabled or disabled. For flow
control enabled ports, by default all frames are treated as lowest priority during transmission scheduling. This is
done so that those frames are not exposed to the WRED Dropping scheme. Frames from flow control enabled
ports feed to only one queue at the destination, the queue of lowest priority. What this means is that if flow
control is enabled for a given source port, then it can guarantee that no packets originating from that port will be
lost, but at the possible expense of minimum bandwidth or maximum delay assurances. In addition, these
"downgraded" frames may only use the shared pool or the per-source reserved pool in the FDB; frames from
flow control enabled sources may not use reserved FDB slots for the highest six classes (P2-P7).
Temporary
Reservation R
TMP
Per-Source Reservations 8-R
1G
Per-Class
Reservations
R
P7
, R
P6
,...R
P2
Shared Pool S