CANOGA PERKINS 9145ELB NID Software Version 4.01 - Manual (Page 99)

CoS Configuration 9145ELB NID Software User’s Manual
9145ELB Queuing Functionality
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Chapter 9
CoS Configuration
9.0 Class of Service
CAUTION: When CoS Support settings are set to Enabled, disabling may lead to tempo-
rary packet loss and/or packet mis-ordering.
When throughput exceeds the available bandwidth, Class of Service (CoS) mechanisms allow
the 9145ELB to selectively queue frames based on priority of the packets serviced. CoS
processing occurs in the egress direction of each port on the NID, that is, for frames transmitted
from a NID port.
From the Class of Service Configuration screen (Figure 9-3), you can enable or disable CoS
support, including Classification, Queuing, Scheduling, and Queue Management functions.
9.1 9145ELB Queuing Functionality
Figure 9-1 shows a logical model for the CoS function on the 9145ELB. With this model, packets
enter the NID on an Ingress Port (for example, 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).