Tuesday, March 24, 2009

CCIE ALU - FRTS and how to compute ts

Foreward: A CCIE ALU is an Autonomous Learning Unit, you can read this post to learn more about them.





Users have been reporting an increase in the number of dropped packets on the frame relay link between R1 and R2.  Your frame relay provider has sent you a notice that they are seeing significantly more traffic attempting to go through the frame relay link then permitted.  You have purchased 128kbit/sec of guaranteed bandwidth from the provider, but there is currently no restriction on the amount of traffic permitted out the frame relay link.  Limit the amount of traffic traversing the frame relay link to the permitted amount.  You will soon be allowing voice traffic across that link, so you must minimize delay and jitter.




Highlight below for the solution:



map-class frame-relay FRTS

 frame-relay cir 128000

 frame-relay bc 1280

 frame-relay be 0


interface Serial0/0

 frame-relay traffic-shaping

 frame-relay class FRTS



R2#show frame pvc 201


PVC Statistics for interface Serial0/0 (Frame Relay DTE)




  input pkts 5             output pkts 5            in bytes 520      

  out bytes 520            dropped pkts 0           in pkts dropped 0        

  out pkts dropped 0                out bytes dropped 0        

  in FECN pkts 0           in BECN pkts 0           out FECN pkts 0        

  out BECN pkts 0          in DE pkts 0             out DE pkts 0        

  out bcast pkts 0         out bcast bytes 0        

  5 minute input rate 0 bits/sec, 0 packets/sec

  5 minute output rate 0 bits/sec, 0 packets/sec

  pvc create time 1w6d, last time pvc status changed 00:00:01

  cir 128000    bc 1280      be 0         byte limit 160    interval 10 

  mincir 64000     byte increment 160   Adaptive Shaping none

  pkts 0         bytes 0         pkts delayed 0         bytes delayed 0       

  shaping inactive   

  traffic shaping drops 0

  Queueing strategy: fifo

  Output queue 0/40, 0 drop, 0 dequeued




Frame relay traffic shaping is the method of controlling the rate at which packets are allowed to exit an interface.  Since frame relay is a multi-access medium, FRTS specifically allows us to specify separate queues for each DLCI, therefore if you need to allow a certain amount of bandwidth between one set of routers, and a separate amount of bandwidth between another set of routers, FRTS is a great option.


By default there is no limit to the amount of traffic exiting an interface, therefore in this case we will be attempting to send traffic up to the full interface speed out the frame relay interface.  Since our service provider is limiting us to 128kbit/sec, it is fruitless to send more than that.


The most important part of this question is to understand how CIR, Bc, Be, and Tc relate mathematically.    In this case, since we are asked to minimize delay and jitter we need to set our Tc as low as possible, which is 10ms.


You cannot set the Tc for FRTS explicitly, therefore we must calculate it.  To obtain the Tc we want, we need to first figure out how many segments should be sent in a single second.  To do this we take 1000 ms and divide it by the desired TC:


1000 / 10 = 100


Now we take our CIR and divide it by the number of segments we will send in a given second to figure out how many bits we should send each time segment, that number will be our Bc:


128000 / 100 = 1280


FRTS will now send 1280 bits every 10ms, resulting in 128,000 bits being sent per second.


It is interesting to note that a router cannot actually send data at less than the interface speed.  Therefore, if our frame relay connection above is a 1.536Mbit/sec T1, FRTS will send 1280 bits in  only 0.84ms, then sit idle for 9.16ms, then repeat that process 99 more times per second, resulting in the equivalent of 128Kbit/sec.


If you remember the above two step process, you will always be able to calculate the Bc needed to achieve a certain Tc.

  1. Figure out how many segments should be sent in a single second    (1000ms / 10ms = 100 segments)
  2. Divide our CIR by the number of segments to send in a single second to calculate Bc (128000bps / 100 segments = 1280 bits/segment)



DocCD items to reference:

Configuration Guide (Configuring Frame Relay Traffic Shaping)

map-class frame relay

frame relay traffic-shaping

frame relay class

show frame-relay pvc

No comments:

Post a Comment