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interface bandwidth and traffic shaping approach


In a FR scenario (hub and spoke topology) at hub side A I have two pvc each one for the two remote sides B and C.

Hub side A has line speed 2Mbps, one spoke side B has line speed 2mbps and the other C has line speed 64kbps.

Both pvcs cir are 48kbps.

Now on one pvc, suppose AB, we need to have voip and data traffic and on the other pvc AC only data traffic.

We will use LLQ, FR traffic shaping and the other QoS features.

For AB pvc traffic classification:

class-map voip

match access-group 100


policy-map qos-AB

class voip

priority 14

class class-default



map-class frame-relay pvc-AB

no frame-relay adaptive-shaping becn

frame-relay cir 48000

frame-relay bc 480

frame-relay be 0

frame-relay mincir 48000

service-policy output qos-AB

frame-relay fragment 60


For pvc AC traffic classification:

class-map dati-AC

match access-group 101


policy-map qos-AC

class dati-AC

bandwidth 36 (75% of mincir)


map-class frame-relay pvc-AC

frame-relay adaptive-shaping becn

frame-relay cir 64000

frame-relay bc 6000

frame-relay be 2000

frame-relay mincir 48000

service-policy output qos-AC

frame-relay fragment 60



In first case AB we use a "conservative approach" : no adaptive shaping and cir=mincir=48kbps.

So the FR subinterface for the pvc AB has a "well know" bandwidth and if I configure the statement "bandwidth 48" in subinterface I think it could not be ambiguous.

interface s0.20 point-to-point

description pvc AB

bandwidth 48

In second case pvc AC, we don't use a conservative approach so traffic can exceed mincir and subinterface bandwidth isn't "well know" !

In queues and traffic flows bandwidths/weigths calculation, LLQ/WFQ algorithm uses, if configured, the mincir to determine the available bandwidth.

The "bandwidth" statement in interface configuration has other important uses for example in interface load calculation, dynamic routing....


For pvc AC subinterface where approach is NOT conservative and bandwidth fluctuates, the use of "bandwidth 48" (mincir) statement

interface s0.50 point-to-point

description pvc AC

bandwidth 48

could be AMBIGUOUS for queues and traffic flows bandwidths/weigths calculation or it is mandatory ?


Re: interface bandwidth and traffic shaping approach

1. You need to add CIR to your frame-relay maps

not micCIR. MinCIR is only looked at when we're doing adptive

shaping for refereal to drop down level.

2. If you add a bandwidth statement on an interface, it has nothing to do

with the actual frame forwarding over the DLCI, its mostly for routing protocol

use. The traffic queue is kick off by conjection which is measured against CIR and

not interface bandwidth statement.

3. It is mandatory that you have a CIR, BC configured for mapp-class frame-relay when doing

traffic shaping.

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