Re: Q. on frame relay fragmentation

From: Petr Lapukhov (petr@internetworkexpert.com)
Date: Mon Oct 02 2006 - 01:31:41 ART

Let's talk a little bit more about FRF.12, leaving VoFR stuff aside :)

To start with, there are two ways of configuring FRF.12 - per VC and at
interface level.

In both cases, important moment is that fragmentation occurs after
dequeueing
(but _before_ intereaving). And fragmentation decision is based solely on
packet *size*, nothing else. (At least, on hardware platforms commonly
encountered in CCIE lab, check out Chris Lewis' reply a bit later)

Now, there is no way to _conditionally_ avoid packet fragmentation with
FRF.12,
even by putting them into any kind of priority queue. This has been
discussed
a few times, just a couple of references:

http://www.groupstudy.com/archives/ccielab/200605/msg01632.html
http://www.groupstudy.com/archives/ccielab/200605/msg01675.html

To finish, let's just add that whole idea of fragmentation is connected to
the
concept of "interleaving" queue. With per-VC this is dual-FIFO at interface
level.
With interface level fragmentation it's unclear, but Cisco says it works :)

HTH

PS
There is also exist "voice-adaptive" fragmentation, that turns on FRF.12,
based on packets presense in priority queue. But it still uses packet size
for fragmentation decision.

2006/10/2, Scott Morris <swm@emanon.com>:
>
> That depends on what you do with them. FRF11 is designed for VoFR, and
> will
> not fragment any VoFR frames. FRF12 is more generic in nature, and makes
> no
> specific delineation for voice or other packet types (IP is IP).
>
> However, if you have voice in a priority (LLQ) queue on the interface,
> anything in the priority queue will bypass the entire fragmentation scheme
> and is therefore unaffected. There are some previous posts regarding this
> if you want to search the archives for a little more detail that may be
> slipping my mind at the moment.
>
> HTH,
>
>
> Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713, JNCIE
> #153, CISSP, et al.
> CCSI/JNCI-M/JNCI-J
> IPExpert VP - Curriculum Development
> IPExpert Sr. Technical Instructor
> smorris@ipexpert.com
> http://www.ipexpert.com
>
>
> _____
>
> From: sabrina pittarel [mailto:sabri_esame@yahoo.com]
> Sent: Sunday, October 01, 2006 3:37 PM
> To: Scott Morris; ccielab@groupstudy.com
> Subject: Re: Q. on frame relay fragmentation
>
>
> I see...
> are then voice packets automatically excluded from fragmentation or should
> I
> manually set the fragment size to be bigger then the voice packets...is
> the
> this behavior different for different types of fragmentation (FRF.12
> FRF.11
> or cisco)?
>
> Sabrina
>
>
> ----- Original Message ----
> From: Scott Morris <swm@emanon.com>
> To: sabrina pittarel <sabri_esame@yahoo.com>; ccielab@groupstudy.com
> Sent: Sunday, October 1, 2006 11:23:10 AM
> Subject: RE: Q. on frame relay fragmentation
>
>
> Because it will introduce extra latency into the mix, and voice traffic is
> latency-sensitive. So you don't want to have choppy voice.
>
> That and voice packets are typically small enough that it makes little
> sense
> to fragment them. Fragmentation is done to make all things play better
> with
> each other on slower speed links. And since voice (or SNA or some other
> stuff) is generally the reason for fragmentation, it wouldn't serve much
> purpose to fragment that traffic as well.
>
> HTH,
>
>
> Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713, JNCIE
> #153, CISSP, et al.
> CCSI/JNCI-M/JNCI-J
> IPExpert VP - Curriculum Development
> IPExpert Sr. Technical Instructor
> smorris@ipexpert.com
> http://www.ipexpert.com
>
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> sabrina pittarel
> Sent: Sunday, October 01, 2006 2:17 PM
> To: ccielab@groupstudy.com
> Subject: Q. on frame relay fragmentation
>
> Hi,
> I was reading about frame relay fragmentation and everywhere in the
> documentation I read that voice packets should not be fragmented...
> Why is
> that?
>
> Sabrina
>
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-- 
Petr Lapukhov, CCIE #16379
petr@internetworkexpert.com

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