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New Member

Fragmen using FRF.12


I'm trying to vonfigure a VoIP over FR network. I installed two devices (3640 & 2610) using FXS ports and the next FR map: (is from a CCO sample we used)

map-class frame-relay fr-class-voip

no frame-relay adaptive-shaping

frame-relay cir 64000

frame-relay bc 1000

frame-relay be 0

frame-relay mincir 64000

frame-relay fair-queue

frame-relay fragment 160

frame-relay ip rtp priority 16384 16383 48

When I connect more devices we experience some voice problems. could anybody help me whit this?.

Our first question is how do I calculate the Fragmen size for a PVC of 128K with a CIR of 64?.. and for the case of 64K with a CIR of 32k?... finally, how do I calculate the frame-relay ip rtp priority 16384 16383 48?

New Member

Re: Fragmen using FRF.12


You have to calculate the fragment-size with the mincir value. You will use on all you routers the fragment-size of the slower link. There is a table on CCO where you can find the fragment-size value for all slow-speed links. For example the fragment-size for a 64Kbps line is 80 and for a 128Kbps line is 160.

Don't forget also to implement traffic shaping into your configuration.



Re: Fragmen using FRF.12

Fragmentation size calculation:

Lowest Link Speed in Path Recommended Fragmentation Size

56Kbps 70 bytes

64Kbps 80 bytes

128Kbps 160 bytes

256Kbps 320 bytes

512Kbps 640 bytes

768Kbps 1000 bytes

1536Kbps 1600 bytes


The theory behind IP RTP Priority is that VoIP traffic is identified by the fact that it uses a well-known UDP port range (16384-32767)


frame ip rtp priority

frame ip rtp priority 16384 16383 48

This command allows udp ports 16384-32767 to have priority with a maximum bandwidth of 48K. Depending on what bitrate codec you are using you might be exceeding this.

Hope this helps you out.

Don Broks

Cisco Employee

Re: Fragmen using FRF.12

If you're having problems, I would try to use something like 77 as the fragmentation size. Make sure you have frame-relay traffic-shaping enabled on the main serial interface. Also, make sure that if there are other PVCs, they should be shaped, too. Could you explain the voice problems? I assume you're hearing breakup, but more importantly, which side is hearing it (the 3600, the 2600 or both)? That will help determine which side the problem may lie. The configuration is fine, from what you've shown, but chances are there are other PVCs or something else out there. At how many calls do you start experiencing problems?