The BT Business routers actually look at the connection ADSL/VDSL and decide what to set it at.

Dont you mean PC should be 8 higher than router to cater for the 8 bytes added by router?

So 1492 at router and 1500 at PC.

Many routers, not just BT, max at 1492; many OSs default to 1500.

Router makers would not impose this limit if the maths were your way round.

In reply to a post by RobertoS:

Hmmm ?

My Buffalo is set to its default of 1500. I'm not aware of a problem, but perhaps I have one I'm not aware of?

What downside could this be causing please?

If your PC's MTU is higher than the MTU of the router, then you send large packets to the router that the router has to fragment before sending on to the DSL line. This gives the router CPU more work to do than if the packets originate at the correct size.

Windows Vista, and 7 have automatic Path-MTU adjustment, so *should* be recognising this automatically and adjusting.

1500 is the max MTU size for ethernet (without jumbo frames) which is why operating systems come set with this value at the start.

Corporates who use VPNs often lower the MTU on the network cards (wired & wireless can be separate) to allow for the encapsulation of the packet.

So on Win 7 the downstream should be fine, but messy on upstream? My standby XP machine needs checking?

Edited by RobertoS (Mon 16-Jul-12 08:38:40)

In reply to a post by XRaySpeX:

Dont you mean PC should be 8 higher than router to cater for the 8 bytes added by router?

No. I think you're misunderstanding MTU- it's the maximum size of a frame that the device can handle or create.

So if the PC sends a frame which is larger than the setting of MTU on the router, the router will need to fragment it before sending it on. This will slow things down, partly because more frames (hence higher overhead/payload ratio) will need to be sent and partly because the router has to do more work.

To avoid this, the basic rule that the PC MTU should not be larger than the router MTU is valid, but an extension of that is to allow for any extra that the router may need to add. Not a problem for PPPoA because it doesn't.

All that goes out the window if the router (and the ISP) can handle jumbo frames, but not all of them can so it's safest to assume that they don't.

In reply to a post by Ripley:

Still interested to know why the maximum packet size would reduce when this problem occurs for me, then a reboot of the Openreach FTTC modem fixes the problem...

No idea about the first, but rebooting the modem will drop the connection so a new session has to be authenticated, negotiated and set up from scratch. Whether that is a clue I couldn't say, but next time it happens try manually disconnecting and reconnecting via the router interface to see if it has the same effect.

If it does then it would appear to be something related to the duration of the session, though I've no idea how, why or at which end the problem lies

Oops, forgot this bit (more coffee needed...)

In reply to a post by XRaySpeX:

Many routers, not just BT, max at 1492

Maybe so, but it doesn't change the rules- in order to avoid fragmentation the PC MTU should not be higher than the router MTU.

You'll have to ask the router manufacturer/supplier if you want to know why they do it.

many OSs default to 1500

That's the maximum allowed by Ethernet, so fair enough.

Edited by billford (Mon 16-Jul-12 09:14:25)

In reply to a post by RobertoS:

So on Win 7 the downstream should be fine, but messy on upstream? My standby XP machine needs checking?

Shouldn't be - there is only a single MTU per adapter.

MTU on my router is set to 1454 - I've never changed it to my knowledge. Maximum size from the ping before fragmentation is 1426, which makes sense. On my PC network adapter it's set to 1500. Should I be setting the router to 1492 (maximum allowed) and the PC to 1464? Does it really matter? I'm not experiencing any problems AFAIK.

Edited by kasg (Mon 16-Jul-12 09:56:07)

In reply to a post by kasg:

Should I be setting the router to 1492 (maximum allowed) and the PC to 1464?

Playing with the values might be worth a try, but I'd leave the router at 1454 (because of the max before fragmentation of 1426 that you've found) and drop the PC to 1446 (1454 minus 8). And see if anything improves.

If it doesn't then put everything back the way it was, following the even more basic rule that if it ain't broke, don't fix it

Your low maximum value on the router may be due to something else in the routing that doesn't like "full size" frames.