Mar
23

This blog post is taken from the INE Resources area Understanding Frame-Relay Traffic Shaping presentation by Brian Dennis.

Overview

Frame-Relay traffic shaping is designed to control the amount of traffic the router sends out of an interface or out of a particular DLCI. Common reasons for Frame-Relay traffic shaping are:

  • It allows the router to conform to the rate subscribed with the service provider
  • It allows for the throttling of a higher speed site (768K) so that it does not overrun a lower speed site (64K)

Traffic shaping is designed to delay excess traffic, whereas policing is designed to drop excess traffic.

Terminology

  • Available Rate (AR) - the actual physical speed of the interface; on a DCE serial interface this is determined by the configured clock rate. On a DTE serial interface, it is determined by the received clock rate. A router will always (by default) try to send out at the AR regardless of the interface bandwidth. AR is also commonly referred to as port speed, line rate, or access rate.

  • Committed Information Rate (CIR) - the rate the device will send at (on average) over a one second period. The default CIR when traffic-shaping is enabled on the interface is 56K. CIR is also referred to as the "target rate". Since the device is forced to send at the AR, it does not send all of the time (within one second)  in order to send an average amount of data that equals the CIR.
  • Minimum CIR (mincir) - the rate the service provider guarantees to accept. Theoretically, the provider will set the DE bit for all traffic above this rate. Mincir is designed to be used in conjunction with adaptive shaping. With adaptive shaping, the router will throttle down in the event of congestion. The router will not throttle down below this value.
  • Committed Burst (Bc) - the number of committed bits allows to be sent during a given interval. The device sends an average amount of traffic to achieve the CIR. The Bc value defaults to 1/8 of the configured CIR for speeds below 650K. For speeds above that, it is roughly 1/16 of CIR.
  • Excess Burst (Be) - the number of non-committed bits the router is allowed to send above Bc during the first interval (Tc). The amount of Be "credits" is derived from unused Bc credits in previous intervals. There is no limit to how long Be can "store" unused Bc credits. It is a common misconception that Be can only store credits from the previous interval or the previous second. There is no default Be value.
  • Committed Rate Measurement Interval (Tc) - the time interval over whic Bc or Bc+Be can be transmitted. The max value is 125 ms and the minimum value is 10 ms.

The Formula

CIR, Tc, and Bc are related mathematically by the following formula:

CIR = Bc/(Tc/1000)

Notice the division of Tc by 1000 is used to convert milliseconds into seconds - the common measurement of CIR and Bc.

Example 1

Task:

The port speed of your serial interface is 192Kbps. You must shape the interface to send at 128Kbps.

Solution:

interface serial 0/0
encapsulation frame-relay
frame-relay traffic-shaping
frame-relay class MYCLASS
!
map-class frame-relay MYCLASS
frame-relay cir 128000

Example 2

Task:

The port speed is 384K. The router should send at a rate of 192K. The router should throttle down to 128K upon the receipt of BECNs. Allow the router to burst up to port speed.

Solution:

interface serial 0/0
encapsulation frame-relay
frame-relay traffic-shaping
frame-relay class MYCLASS
!
map-class frame-relay MYCLASS
frame-relay cir 192000
frame-relay be 24000
frame-relay mincir 128000
frame-relay adpative-shaping becn

Example 3

Task:

The port speed is 384K. The router should send at a rate of 128K. The router should throttle down to 64K upon the receipt of BECNs. Allow the router to burst up to port speed. Use a timing interval of 100 ms.

Solution:

interface serial 0/0
encapsulation frame-relay
frame-relay traffic-shaping
frame-relay class MYCLASS
!
map-class frame-relay MYCLASS
frame-relay cir 128000
frame-relay bc 12800
frame-relay be 25600

frame-relay adpative-shaping becn

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