42-120
Cisco7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
OL-4266-08
Chapter42 Con figuring PFC QoS
Common QoS Scenarios
The WRR algorithm uses relative weights that are assigned to the WRR queues. If there are three queues
and their weights are 22:33:45 (which are the default settings), then queue 1 gets only 22percent of the
available bandwidth, queue 2 gets 33 percent, and queue 3 gets 45 percent. With WRR, none of the
queues are restricted to these percentages. If queue 2 and queue 3 do not have any traffic, queue 1 can
use all available bandwidth.
In this example, queue 1 has a lower priority than queue 2, and queue 2 has a lower priority than queue3.
The low-priority traffic (phone-other and PC-other) maps to queue 1, and the medium-priority traffic
(voice-signaling and PC-SAP) maps to queue 2.
The strict-priority queue does not require any configuration after traffic has been mapped to it. The WRR
queues have a default bandwidth allocation that might be sufficient for your network; if it is not, then
you can change the relative weights to suit your traffic types (see the “Allocating Bandwidth Between
Standard Transmit Queues” section on page42-108).
The best way to verify that the router is handling oversubscription is to ensure that there is minimal
packet drop. Use the show queueing interface command to determine where that packet loss is
happening. This command displays the number of dropped packets for each queue.
Using Policers to Limit the Amount of Traffic from a PCRate limiting is a useful way of ensuring that a particular device or traffic class does not consume more
bandwidth than expected. On the Cisco 7600 series router Ethernet ports, the supported rate-limiting
method is called policing. Policing is implemented in the PFC hardware with no performance impact. A
policer operates by allowing the traffic to flow freely as long as the traffic rate remains below the
configured transmission rate. Traffic bursts are allowed, provided that they are within the configured
burst size. Any traffic that exceeds the configured rate and burst can be either dropped or marked down
to a lower priority. The benefit of policing is that it can constrain the amount of bandwidth that a
particular application consumes, which helps ensure quality of service on the network, especially during
abnormal network conditions such as a virus or worm attack.
This example focuses on a basic per-interface aggregate policer applied to a single interface in the
inbound direction, but you can use other policing options to achieve this same result.
The configuration of a policer is similar to the marking example provided in the “Classifying Traffic
from PCs and IP Phones in the Access Layer” section on page 42-113 because policing uses the same
ACL and MQC syntax. The syntax in that example created a class-map to identify the traffic and then
created a policy-map to specify how to mark the traffic.
The policing syntax is similar enough that we can use the marking example ACL and modify the marking
example class map by replacing the set dscp command with a police command. This example reuses the
CLASSIFY-OTHER class-map to identify the traffic with a modified IPPHONE-PC policy map to police
the matched traffic to a maximum of 50Mbps, while continuing to mark the traffic that conforms to this
rate.
The class maps and the ACL and class-map commands that are used to identify the “other” traffic are
included below for reference; no changes have been made.
•ACL commands:
ip access-list extended CLASSIFY-OTHER
permit ip any any
•Class map commands:
class-map match-all CLASSIFY-OTHER
match access-group name CLASSIFY-OTHER