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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: New MAE-EAST
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 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).
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