QoS and QoS Policies
52
Quality of Service Guide
3HE 11014 AAAC TQZZA Edition: 01
In Figure 8, the top half is identical to access egress traffic management, where CoS
queues (Tier 1) feed into either per-SAP shapers for shaped SAPs (#1) or a single
second-tier shaper for all unshaped SAPs (#2). Up to the end of the second-tier,
per-SAP aggregate shapers, the access egress datapath is maintained in the same
manner as an Ethernet port in access mode. The same logic applies for network
egress. The bottom half of the figure shows the datapath from the CoS queues to the
per-VLAN shapers, which is identical to the datapath for any other Ethernet port in
network mode.
The main difference between hybrid mode and access and network modes is shown
when the access and the network traffic is arbitrated towards the port (Tier 3). At this
point, a new set of dual-rate shapers (called shaper groups) are introduced: one
shaper for the aggregate (bulk) of the access traffic (#3) and another shaper for the
aggregate of the network traffic (#6) —to ensure rate-based arbitration among
access and network traffic.
Depending on the use and the application, the committed rate for any one mode of
flow might need to be fine-tuned to minimize delay, jitter and loss. In addition, through
the use of egress-rate limiting, a fourth level of shaping can be achieved.
When egress-rate is configured (under config>port>ethernet), the following events
occur:
• egress-rate applies backpressure to the access and network aggregate
shapers
• as a result, the aggregate shapers apply backpressure to the per-SAP and
per-VLAN aggregate shapers
− access aggregate shapers apply backpressure to the per-SAP aggregate
shapers and the unshaped SAP aggregate shaper
− network aggregate shapers apply backpressure to the per-VLAN aggregate
shapers and the unshaped VLAN aggregate shaper
• as a result, the per-SAP and per-VLAN aggregate shapers apply backpressure
to their respective CoS queues
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