MVTX2801
Data Sheet
Temporary
Reservation RTMP
Per-Class Reservations
Shared Pool S
R
p7, Rp6 ... Rp2
Per-Source Reservations 8 . R1G
Figure 4 - Buffer Partition Scheme Used in the MVTX2801
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 MVTX2801 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 Flow Control Basics
Because frame loss is unacceptable for some applications, the MVTX2801 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 MVTX2801, 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 we 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).
The MVTX2801 does provide a system-wide option of permitting normal QoS scheduling (and buffer use) for frames
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