Broadband Related > ADSL Issues

MTU in router or PC .... which one is important

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setecio:
Routers have an MTU setting and PCs also have an MTU setting.

Which one overrides the others, which is the important one ...... eg if the router is set to 1500 and the PC to 1432, this would seem to be fine and I expect the internet to work on the MTU of 1432.

However if the PC is set to 1500 and the router to 1432, will the PC send at 1500 causing problem at the router where 1432 is the max ?

roseway:
Set the MTU of the PC to the required value, and either set the router to the same value or (more simply) leave it at 1500. If you set the router to a lower value but leave the PC at 1500, the connection will work, but outgoing packets will be fragmented by the lower router value.

setecio:
OK thanks Roseway.

setecio:
It suddenly occurred to me about AOL's requirement to set router MTUs to 1400. Is this only important if you are on AOL and not using the AOL browser since it would surely be common sense of AOL to have  setting in their software which sets tht PC's MTU at 1400. Does anyone know if the AOL browser does set the MTU of packets from it at 1400? If not, why not ?

kitz:
AOL (and TT) users do appear to be those that suffer more frequently from MTU issues for obvious reasons.  Eric is correct when he says that MTU should be set on the PC rather than the router as the router can be the point where incoming packets become fragmented if the MTU on the router is lower than on the local PC.

It can also be dependent on whether the router is blocking pings.  If you make your router as unpingable, then this stops ICMP totally.  Turning off ICMP breaks Path MTU Discovery.

Therefore AOL users not using a router*, or a router that is pingable are unlikely to notice the blackholing effect that can occur, since Path MTU Discovery will do its job correctly and adjust the end to end packet size accordingly.


*Some software firewalls on local machines could also block ICMP.
Despite what the likes of GRC says, theres many which say that turning off ICMP is bad and it "breaks the internet".  So what if your router is pingable?

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