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ATM Switch Router Software Configuration Guide
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Chapter22 Configuring Quality of Service
About IP QoS on the Enhanced Gigabit Ethernet and Enhanced ATM Router Module Interfaces
Note There must be at least one traffic conditioning element associated with every classifier in an input policy
map.
Per Hop Behavior Definition
Per Hop Behavior or PHB is the externally observable forwarding behavior (in terms of
buffer/bandwidth resource allocation), applied to a particular traffic class. This is essentially defined by
the queuing/scheduling/buffer management in the forwarding path.
Queuing
Once the traffic stream is classified and conditioned, the forwarding engine is consulted to get the
destination interface to which the packet needs to be switched. There are four output queues for each
physical interface and each can be assigned to an output traffic class. A direct lookup table, called the
queue selector table, is used to determine which is the correct queue for the pa cket. This table is indexed
using a combination of the output interface and DSCP from the packet header.
All entries in this table are initialized to 0 by default (Q0 is the queue for best effort behavior). This
mapping may be changed through user configuration.
Figure22-2 Four Queues Per Physical Interface
Physical Interface
Queue 0 Queue 1 Queue 2 Queue 3
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