41-20
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
OL-4266-08
Chapter 41 Configuring PFC QoS
Understanding How PFC QoS Works
Classification and Marking on the MSFC
PFC QoS sends IP traffic to the MSFC with the final internal DSCP values. CoS is equal to
IP precedence in all traffic sent from the MSFC to egress ports.
Figure 41-9 Marking with PFC2 or PFC3 and MSFC2, MSFC2A, or MSFC3
Note Traffic that is Layer 3 switched on the PFC does not go through the MSFC and retains the CoS value
assigned by the PFC.
Policers
These sections describe policers:
• Overview of Policers, page 41-20
• Aggregate Policers, page 41-21
• Microflow Policers, page 41-22
Overview of Policers
Policing allows you to rate limit incoming and outgoing traffic so that it adheres to the traffic forwarding
rules defined by the QoS configuration. Sometimes these configured rules for how traffic should be
forwarded through the system are referred to as a contract. If the traffic does not adhere to this contract,
it is marked down to a lower DSCP value or dropped.
Policing does not buffer out-of-profile packets. As a result, policing does not affect transmission delay.
In contrast, traffic shaping works by buffering out-of-profile traffic, which moderates the traffic bursts.
(PFC QoS does not support shaping.)
144800
To egress
port
IP traffic
from PFC?
Write ToS
byte into
packet
No
Ye s
Multilayer Switch Feature Card (MSFC) marking
From PFC
Route
traffic
CoS =
IP precedence for
all traffic
(not configurable)
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