CANOGA PERKINS 9145E NID Software Version 4.10 - Manual (Page 195)

CoS Configuration 9145E NID Software User’s Manual
9145E Queuing Functionality
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Chapter 10
CoS Configuration
10.0 Class of Service
CAUTION: When CoS Support settings are set to Enabled, disabling may lead to tempo-
rary packet loss and/or packet misordering.
When throughput exceeds the available bandwidth, Class of Service (CoS) mechanisms allow
the 9145E to selectively queue service frames based on priority of the packets. From the CoS
Configuration screen, you can enable or disable CoS support, which includes Classification,
Queuing, Scheduling and Queue Management functions, for the 9145E in the ingress or egress
direction independently. The Ingress direction is from the User Port into the Network (NET). The
reader should note that in this context, the “ingress direction” refers to traffic that flows into the
User Port and out the Network Port. The “egress direction” refers to traffic that flows into the
Network Port and out the User Port.
From the Class of Service Configuration screen (Figure 10-2) you can modify classifier mapping,
queue configuration, and congestion avoidance profiles for ingress and egress paths
independently.
10.1 9145E Queuing Functionality
Figure 10-1 shows a logical model for the CoS function on the 9145E. With this model, packets
enter the NID on an Ingress Port (e.g., a User Port or Network Port). If these well-formed
packets are not discarded by the Port Filtering rules or due to a bad FCS, they are handed to a
Classifier function, which will further inspect the packet. The Classifier looks to assign a CoS
level to a packet by inspecting specific indicators present in the packet header, such as VLAN-ID,
PCP or DSCP/ToS. The Classifier cross-references these indicators with the CoS assignments
in the user configurable Class Map to determine the CoS for the packet, and submits it to the tail
of the appropriate queue associated with the Egress Port. The Queue is simply a FIFO buffer,
with user configurable length. Generally speaking, if an Egress Port supports multiple Classes of
Service, it will have multiple queues assigned to it (i.e., one for each CoS). In an asynchronous
fashion, the Transmission Scheduling mechanism waits for the Egress Port to indicate to it that
the transmission medium (not shown) is ready for another packet to be transmitted. The
Transmission Scheduling function then selects a packet from the head of a queue to transmit out
the Egress Port based on some scheduling algorithm (such as SP or WFQ).