Voice over Frame-Relay

Unanswered Question
Oct 1st, 2007

Hi,

Customer of me has several sites connected with leased lines. The leased lines have frame-relay encapsulation (frame-relay back-to-back)

The line has two pvc's configured, one for voice and one for data. The voice PVC works fine. On the data pvc we want to put some QoS.

Here is the partial configuration:

interface Serial0

bandwidth 2000

no ip address

encapsulation frame-relay

no ip route-cache cef

ip route-cache flow

no ip mroute-cache

frame-relay intf-type dce

!

interface Serial0.1 point-to-point

ip address 10.xx.xx.1 255.255.255.252

no ip mroute-cache

frame-relay interface-dlci 120

!

interface Serial0.2 point-to-point

ip address 10.0.0.1 255.255.255.252

no ip mroute-cache

frame-relay interface-dlci 100

class VOICE

vofr cisco

!

map-class frame-relay VOICE

frame-relay cir 512000

frame-relay voice bandwidth 256000

frame-relay fragment 320

frame-relay fair-queue

Question is can I just apply a service policy out to the data DLCI or doi i also have to enable frame-relay traffic shaping?

Thanks,

JH

I have this problem too.
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Maulik Shah Sun, 11/04/2007 - 07:14

If you want to implement any fancy queueing, this only works if there is some congestion or backpressure. For Frame Relay PVCs as they are virtual links, the backpressure can only be created via traffic shaping. Hence you need to use shaping on the data pvc.

Looking at your config, you already have a map-class for voice, so would suggest you create another one for data, apply the service policy there and enable traffic shaping on the main interface

Paolo Bevilacqua Mon, 11/05/2007 - 03:00

The disadvantage of shaping is that it will actually limit you bandwidth. That is, you won't be able to use the circuit at full capacity, because of the artificial limitation induced by shaping. This is particularly true for a situation like you where the actual circuits is a leased line, and not a FR network.

A better approach would be to reconfigure for simple hdlc encap, and run VoIP instead. This way, you would have all the QOS features you need for voice and data, and be able to use the full capacity with a much simpler configuration.

Actually, even if you want to retain the FR encapsulation and the voice over FR approach, still using a single PVC would be better, for the same reasons given above.

Hope this helps, please rate post if it does!

j.huizinga Mon, 11/05/2007 - 03:43

Hello Paolo,

I know what you mean, but this customer had old 3810 routers that did not understand VoIP, only VoFR.

We have now upgraded the 3810 to 3845 routers, but I don't want to change the configuration, because I installed Callmanager 6 and we shall move to full VoIP.

So it is just a matter of time when we can disconnect the old PBX and remove all the VoFR configuration.

Thanks,

Jan

Paolo Bevilacqua Mon, 11/05/2007 - 04:40

Hi Jan,

thanks for clarifying the background.

In full honesty, I think you have been lucky already with the 3845 working with the VoFR config taken from the 3810s!

Good luck!

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