Noemi Berry wrote:

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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 ethernet... 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 into the ATM fabric. FDDI to ATM to Ethernet, try it , you won't like it. PS Don't 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 using..... 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! ) Sigh........ 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).