25ms minimum Tc in MQC Shaping??

 

Hi guys,

 

i'm just concerned why in newer IOSes, the minimum Tc for shaping is 25ms??

 

R1#sh ver | i IOS

Cisco IOS Software, 1841 Software (C1841-ADVENTERPRISEK9-M), Version 12.4(24)T5, RELEASE SOFTWARE (fc3)

 

R1(config)#policy-map PM_SHAPE_OUT

R1(config-pmap)#class class-default

R1(config-pmap-c)#shape average 256000 2560     

Shaping Interval is 10 milliseconds. Intervals below 25 milliseconds rejected

R1(config-pmap-c)#

 

 

>>Doing shaping without explicitly configuring the Bc or Be results to:

 

R1(config)#policy-map PM_SHAPE_OUT

R1(config-pmap)#class class-default

R1(config-pmap-c)#shape average 256000

R1(config-pmap-c)#exit

R1(config-pmap)#exit

R1(config)#map-class frame-relay PVC_103

R1(config-map-class)#service-policy output PM_SHAPE_OUT

R1(config-map-class)#exit

R1(config)#int se0/0/0.123

R1(config-subif)#frame-relay interface-dlci 103

R1(config-fr-dlci)#class PVC_103

R1(config-fr-dlci)#end

 

R1#sh policy-map int se0/0/0.123

 Serial0/0/0.123: DLCI 103 -

 

  Service-policy output: PM_SHAPE_OUT

 

    Class-map: class-default (match-any)

      7 packets, 484 bytes

      5 minute offered rate 0 bps, drop rate 0 bps

      Match: any 

      Queueing

      queue limit 64 packets

      (queue depth/total drops/no-buffer drops) 0/0/0

      (pkts output/bytes output) 7/484

      shape (average) cir 256000, bc 6400, be 6400

      target shape rate 256000

        lower bound cir 0,  adapt to fecn 0

 

>>which results to a Tc = 6400 bits/256000 bps = 25 ms

 

 

 

>>but in Legacy FRTS, we can still use a minimum Tc of 10ms..

 

R1(config)#map-class frame-relay PVC_103

R1(config-map-class)#frame-relay cir 256000

R1(config-map-class)#frame-relay bc 2560

R1(config-map-class)#int se0/0/0

R1(config-if)#frame-relay traffic-shaping 

R1(config-if)#int se0/0/0.123

R1(config-subif)#frame-relay interface-dlci 103   

R1(config-fr-dlci)#class PVC_103

R1(config-fr-dlci)#do sh traffic-shape int se0/0/0.123

 

Interface   Se0/0/0.123

       Access Target    Byte   Sustain   Excess    Interval  Increment Adapt

VC     List   Rate      Limit  bits/int  bits/int  (ms)      (bytes)   Active

103           256000    320    2560      0         10        320       -  

 

 

>> by the way, in Cisco 12.4T command reference guide for MQC shape command, it says:

"If you do not specify the committed burst size (Bc) and the excess burst size (Be), the algorithm decides the default values for the shape entity. The algorithm uses a 4 milliseconds measurement interval, so Bc is CIR * (4 / 1000)."

 

>> but this is not true when you try it out in the command line, just like what i have posted above..

 

>> but if you try it out on earlier IOS like 12.4(6)T, you can do MQC shaping up to a minimum of 4ms..

 

Anyways, does this mean that if we are given a Configuration task in the lab exam on FRTS stating that "issue a Commited burst rate that will result in best latency for Voice traffic", we should choose Legacy FRTS over MQC FRTS??

 

 

TIA,

Carlos

 

 

 

 

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