The BC,BE dilemma

So it's always been my worst topic of the day , understanding what those values are. Even in live network at work i always use their default values and never tried to modify them, but here i am today and it took me quite  a while to try to somehow understand how those work and below is my simple simplification of the concept which may or may not be correct.

The first way to understand those values is to divide the technologies that use those terms : 

1- Traffic Shaping 
2- Frame-Relay Traffic Shaping
3- Policing
4- CAR

Each of those technologies use those terms BC,BE but each technology use those terms differently.

   Snapshot from Jeremy Cioara Explanation of shaping

1-Traffic Shaping

What's traffic shaping , what traffic shaping does is that it tries to buffer excess packets and maintain a constant traffic rate that's below the customer rate. This is mostly seen on Metroethernet connections for example where the Ethernet speed is 10 Mgb/s and customer only purchased 5 Mgb/s so the only way to stop him is to use traffic shaping under a policy-map
which would be something like this.

policy-map limit-out
class class-default
shape average 1000000 

now what if the customer can burst more than this rate we need to set two values one Bc which is the bytes rate (or in other words the Tc) of the customer and the AIR which is the maximum allowed rate that the customer can send traffic in.

Let's say that the customer AIR would be 8 Mg in this case we would need to do the math. 

AIR = (bc+be)/Tc    ok but what are the Tc and the Bc 

so the point here the Bc is just a translation to tell the router what's its Tc to shape upon and to do the math in case we need to calculate the Be

For example by default

R5(config)#policy-map LIMIT
R5(config-pmap)# class class-default
R5(config-pmap-c)#shape average 1000000



R5#show policy-map int fa1/0 out
 FastEthernet1/0

  Service-policy output: LIMIT

    Class-map: class-default (match-any)
      112858 packets, 157474318 bytes
      30 second offered rate 6612000 bps, drop rate 6505000 bps
      Match: any
      Traffic Shaping
           Target/Average   Byte   Sustain   Excess    Interval  Increment
             Rate           Limit                bits/int  bits/int  (ms)      (bytes)
          1000000/1000000   6250   25000     25000     25        3125   <<<<<<<<<<<< The router use default interval 25 ms 

        Adapt  Queue     Packets   Bytes     Packets   Bytes     Shaping
        Active Depth                         Delayed   Delayed   Active
        -      0         1961      2682860   1157      1606954   no

Now if we want to modify those parameters

R5(config)#policy-map LIMIT
R5(config-pmap)#class class-default
R5(config-pmap-c)#shape average 1000000 1000000 1000000



R5#show policy-map int fa1/0 out
 FastEthernet1/0

  Service-policy output: LIMIT

    Class-map: class-default (match-any)
      112852 packets, 157473902 bytes
      30 second offered rate 12973000 bps, drop rate 12763000 bps
      Match: any
      Traffic Shaping
           Target/Average   Byte   Sustain   Excess    Interval  Increment
             Rate           Limit  bits/int  bits/int  (ms)      (bytes)
          1000000/1000000   250000 1000000   1000000   1000      125000   <<<<<<<<< The router re-adjust its Tc calculations

        Adapt  Queue     Packets   Bytes     Packets   Bytes     Shaping
        Active Depth                         Delayed   Delayed   Active
        -      0         1955      2682444   1157      1606954   no

Cisco recommends not messing around with those values as i just did :)
Important note here Bc,BE are valued in bits

2- Frame Relay Traffic-Shaping

Here the shaping mechanism is nearly the same as previously explained shaping commands except for the fact that the Bc,Be values are defined under the frame-relay map class. and the map-class can be configured under a specific DLCI.

3-,4  Policing & CAR

Here one of the major differences is that Bc,Be values are expressed in Bytes  and here the Bc,Be are a way of expressing excess bytes to policing and not bits to traffic shaping. In most of examples i saw in policing we assume the Tc to be 1 second.

So Bc here refers to the bytes that i can burst to conform with the policing policy.

For example :

police cir 96000 bc 12000 be 6000 conform transmit exceed set-dscp-trans 0 violate-action drop

Here the Tc = 1 since Bc = 960000/8  and we can burst up to 0.5's second worth of traffic and that's because 
be = 6000 byte

so bc,be here express the excess bytes limit and the amount of byte they can exceed

This post is not final i believe this can be considered a first draft to my long struggle with Bc,Be values :)