Cisco Systems OL-4266-08 manual Using Policers to Limit the Amount of Traffic from a PC, 42-120

Chapter 42 Configuring 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 22 percent 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 queue 3. 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 page 42-108).

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 PC

Rate 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 42-113because 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 50 Mbps, while continuing to mark the traffic that conforms to this rate.

The class maps and the ACL and class-mapcommands 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

	Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
42-120	OL-4266-08