Richard Irving rirving at
Wed Dec 10 21:08:37 UTC 1997

Noemi Berry wrote:
> > > > I'd certainly be interested in seeing anything
> >> > anyone has on fragmentation related delays going from a large MTU
> switched
> >> > environment to a small MTU switched environment though.  Overall, I've
> >> > been less-than-pleased with latency when I've been transited that way.
> >> >
> >
> >        Uh... Doctor , it hurts when I do this.
> >Doctor: So don't do that!
> >
> >    Hint, it is *pretty* foolish to run mixed MTU's on switched fabric.
> >  *Most* people on ATM exchanges run with common MTU's.
> Getting to my NANOG mail kinda late here....

   I will say.  *grin*

> But WHAT are you guys talking about WRT to MTUs through an ATM switch?
> The MTUs almost always refer to the layer 3 and up MTUs, which the ATM
> switches themselves have no visibility to. 

 We are discussing Router ATM cards. (Layer 2/3)....

> It's righter to say that
> most people on *any* exchange run with common MTUs.

   Don't I wish.

> If you're going from a "large MTU switched environment" to a "small MTU
> switched environment", then you must be passing through a router or some
> other device that understands and sets its own MTU.  The ATM layer has
> nothing to do with that.  MTU issues are independent of what layer 2
> technology is used to connect the peers.

   The point ? You would have a hard time setting an MTU of 4470 on
The object of the game ? If you must fragment, do it *after*
transmission. (If possible)
(Less WAN bandwidth than otherwise) 

> I know a thing or two about ATM, but not NAPs, so please correct me if my
> terminology is screwy and I missed something.

   Try reading RFC-1626.  We are talking about MTU for the IP PLCP card
the ATM fabric. FDDI to ATM to Ethernet, try it , you won't like it. PS
forget ATM is not the only switched fabric in town....

   You will find there is a whole PDU and SDU maxsize declaration for
SAR to use on
AAL5 based framing. However, these are *not* the limits you want to be

   I know non-mixed MTU's seem obvious, but some don't get it. On the
other hand,
I have neighbors who have the CHI-NAP fabric declared as /24, 
and *cannot* be made to understand why this is bad.   ;)

 (One Quote: Its the internet, I can do as I want! )

   Cheers !

> thanks,
> noemi
> --
> Noemi Berry
> Network Operations
> Covad Communications
> p.s. (all that said, it's not *entirely* true that the ATM layer has no
> visibility to the higher-layer MTUs: early- and partial- packet discard peeks
> into the ATM cells to see the start and end of AAL5 frames that encapsulate
> layer 3 packets, but it does no fragmentation nor otherwise manipulate the
> layer 3 packet).

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