Interesting FRTS Behavior Driving Me Mad

Has anybody seen anything like this before? It seems the formula for how my
router is calculating Tc dynamically changes based on the given value of Bc
which I have not read about in any book or cisco doc as of yet.

R2(config-map-class)#do sh run int s0/1/0 | i frame-relay
 encapsulation frame-relay
 frame-relay class foo
 frame-relay traffic-shaping
 frame-relay map ip 100.100.100.5 205 broadcast
 no frame-relay inverse-arp

R2(config-map-class)#do sh run | beg map-class
map-class frame-relay foo
 frame-relay cir 768000
 frame-relay bc 80007

With this setup, I expect the long standing formula Tc = Bc / CIR to hold
true. Therefore Tc should be equal to 80007 / 768000 = 0.10417578125
seconds = 104ms rounded down and indeed it is

R2(config-map-class)#do sh traffic

Interface Se0/1/0
       Access Target Byte Sustain Excess Interval Increment Adapt
VC List Rate Limit bits/int bits/int (ms) (bytes)
Active
205 768000 10000 80007 0 104 9984 -

However, as soon as I cross the Bc boundary of 80007 and set it to 80008 my
Tc is changed to what appears to be (Bc / CIR) / 2 !!! WTF?

R2(config-map-class)#do sh traffic

Interface Se0/1/0
       Access Target Byte Sustain Excess Interval Increment Adapt
VC List Rate Limit bits/int bits/int (ms) (bytes)
Active
205 768000 5000 80008 0 52 4992 -

As you can see the Tc has been set to 52 ms which just so happens to be 1/2
of the previous value of 104ms. Any clues?

-- 
Regards,
Joe Astorino CCIE #24347 (R&S)
Sr. Technical Instructor - IPexpert
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