27-13
Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 27 Configuring QoS
Understanding QoS
EoMPLS and QoS
EoMPLS supports QoS by using three experimental bits in a label to determine the priority of packets.
To support QoS between label edge routers (LERs), you set the experimental bits in both the virtual
connection and the tunnel labels. EoMPLS QoS classification occurs on ingress, and you can match on
Layer 3 parameters (such as IP or DSCP), and Layer 2 parameters (CoS). See the
“Configuring MPLS
and EoMPLS QoS” section on page 27-53
for more information about EoMPLS and QoS.
Policing
After a packet is classified and assigned a QoS label, you can use policing, as shown in
Figure 27-5
, to
regulate the class of traffic. The policing function limits the amount of bandwidth available to a specific
traffic flow or prevents a traffic type from using excessive bandwidth and system resources. A policer
identifies a packet as in or out of profile by comparing the rate of the inbound traffic to the configuration
profile of the policer and traffic class. Packets that exceed the permitted average rate or burst rate are out
of profile or nonconforming. These packets are dropped or modified (marked for further processing),
depending on the policer configuration.
All traffic, whether it is bridged or routed, is subjected to a configured policer. As a result, packets might
be dropped or might have the DSCP or CoS fields modified when they are policed and marked.
Note
Input hierarchical service policies are applied to a traffic stream before any other services act on that
traffic. For example, an input hierarchical service policy applied to traffic could change the traffic rate
from above a storm-control threshold to below the threshold, preventing storm control from acting on
the traffic stream.
Figure 27-5
Policing of Classified Packets
You can attach a policy map with a policer only in an input service policy. The switch supports 1-rate,
2-color ingress policing and 2-rate, 3-color policing.
For 1-rate, 2-color policing, you use the police policy-map class configuration command to define the
policer, the committed rate limitations of the traffic, committed burst size limitations of the traffic, and
the action to take for a class of traffic that is below the limits (conform-action) and above the limits
(exceed-action). If you do not specify burst size (bc), the system calculates an appropriate burst size
value. The calculated value is appropriate for most applications.
141153
Receive
Classify
Drop
Queuing,
scheduling,
and shaping
An exceed-action at this
point results in dropped
or reclassified packets.
Packets that conform
to the committed
information rate (CIR)
Packets that exceed
the CIR
