QoS & VoIP

CAR & CIR – What the hell is the difference?

What is the difference?

When we are talking about Committed Information Rate we are usually talking about Traffic Shaping. This is especially true of Frame Relay Traffic Shaping (FRTS). In FRTS we might configure a CIR with the frame-relay cir command:

For example:

interface serial0
 encapsulation frame-relay
 frame-relay traffic-shaping
 frame-relay interface-dlci 200
 class adjust_vc_class_rate
!
map-class frame-relay adjust_vc_class_rate
 frame-relay cir 64000
 frame-relay mincir 32000
 frame-relay adaptive-shaping becn

Here we have a committed information rate of 32k for the virtual circuit represented by DLCI 200 (we’ll have a look at Frame Relay traffic shaping in more detail later). In this case a shaper is set. From your perspective, the CIR is the rate of the virtual circuit according to a business contract. From the service providers perspective it might involve setting up SLA’s and discarding anything above (discard eligible on Frame Relay).

Committed Access Rate is an example of policing. It is rate limiting to the committed access rate. It is implemented with the rate-limit command. For example:

int fa0/0
 rate-limit output 8000 1000 2000 conform-action transmit exceed-action drop

The example of above sets a committed access rate of 8kbps with 1kbps burst and 2kbps max burst.

© 2007 Arden Packeer. All rights reserved. The opinions expressed herein are my own personal opinions and do not represent my employer's view in anyway. This material is not sponsored or endorsed by Cisco Systems, Inc. Cisco, Cisco Systems, CCIE and the CCIE Logo are trademarks of Cisco Systems, Inc. and its affiliates