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Cisco7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
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Chapter42 Configuring PFC QoS
Common QoS Scenarios
These are the three traffic classes in the sample network:
•Voi c e
•High-priority application traffic
•Best-effort traffic
The QoS configuration described in this section identifies and prioritizes each of these traffic classes.
Note If your network requires more service levels, PFCQoS supports up to 64 traffic classes.
These QoS scenarios describe the following three fundamental QoS configurations, which are often a
general part of QoS deployment:
•Classifying traffic from PCs and IP phones in the access layer
•Accepting the traffic priority value on interswitch links between layers
•Prioritizing traffic on interswitch links between layers
These QoS scenarios assume that the network carries only IP traffic and use the IP DSCP values to assign
traffic priority. These QoS scenarios do not directly use IP type of service (ToS) or Ethernet 802.1p class
of service (CoS).
IP packets can carry a priority value, which can be set at various points within the network topology.
Best-practice design recommendations are to classify and mark traffic as close to the source of the traffic
as possible. If traffic priorities are set correctly at the edge, then intermediate hops do not have to
perform detailed traffic identification. Instead, they can administer QoS policies based on these
previously set priority values. This approach simplifies policy administration.
Note • You should develop a QoS deployment strategy for assigning packet priorities to your particular
network traffic types and applications. For more information on QoS guidelines, refer to RFC 2597
and RFC 2598 as well as the various QoS design guides published by Cisco Systems, Inc.
•Do not enable PFC QoS globally and leave all other PFC QoS configuration at default values. When
you enable PFC QoS globally, it uses its default values. These are two problems that exist with the
PFC QoS default configuration:
–
With PFC QoS globally enabled, the default trust state of the Ethernet ports in the system is
untrusted. The untrusted port state sets the QoS priority of all traffic flowing through the router
to the port CoS value (zero by default): all traffic will be zero-priority traffic.
–
With PFC QoS globally enabled, the port buffers are allocated into CoS-based queues and only
part of the buffer is available for zero-priority traffic: zero-priority traffic has less buffer
available than when PFCQoS is disabled.
These problems with the PFC QoS default configuration can have a negative effect on network
performance.
Classifying Traffic from PCs and IP Phones in the Access Layer
The access layer routers have a PC daisy-chained to an IP phone on a 100 Mbps link. This section
describes how to classify voice traffic from the phone and data traffic from the PC so that they have
different priorities.