Re: [nsp] Few questions about ATM connections and ATM Traffic-Shapping

From: Stephen Sprunk (
Date: Mon Apr 08 2002 - 11:23:48 EDT

Thus spake "Tommy" <>
> I have customer [directly] connected to my network via ATM
> OC3 card. Interface configuration on my Cisco:

> ...

First of all, let me say ATM is designed with the assumption VCs are going
across a carrier cloud. Since you have the devices directly connected, a
large part of the ATM service model no longer applies.

Normally, a carrier polices your traffic according to the particular service
level you have purchased. You shape at your egress (ingress to the cloud)
to prevent having packets/cells dropped by the policing mechanism. However,
since you have no carrier, shaping and policing have a nebulous purpose.

> And now I have few questions according this configuration:
> 1) Is any diffrence in using CBR instead of UBR
> in PVC configuration ? I think that 155mbps OC3 link can guarante
> in 100% 4mbit/s traffic. But CBR looks better...

CBR is a fixed cell-rate service. The carrier guarantees you exactly the
cell rate specified, no more or less. Typically, you are policed at exactly
that cell rate.

True UBR service provides no cell-rate guarantee, but also does not police
you in any way. UBR+ service (what you have configured) provides a minimum
guaranteed cell rate, but again does not have any policing.

Which service you choose depends on how you intend to service your customer.

> 2) Is any _good_ method to calculate Cell Rate for IP
> Mbit/s ? For example I'd like to calculate what Cell Rate is
> "true" 4mbit/s of IP packets for ATM PVC circut with
> aal5snap encapsulation ?

That depends on what you want to figure in for cell tax. My favorite
estimate is 23%, which comes out to 25,625 cells per true Mb/s of IP.

> 3) The cell rate set by CBR/UBR command on my PVC is only responsable
> for outbound traffic (packets transmitted to Customer) ? If
> yes, Can I traffic-shape inbound traffic (packtes transmitted from
> Customer) on this PVC ? Maybe some QOS rate-limit command ?

Shaping is an outbound function, not an inbound function. The inbound side
applies policing (thus encouraging the sender to shape properly).

> 4) If between My router and Customer's router will
> be added ATM Switch. Where traffic-shaping must be done ?
> Still on Routers or only on ATM switch ? Or maybe on Routers
> and ATM Switch ?

Shaping should be done on both routers, and the switch should be configured
to police.


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