Adam Bower wrote:
MTU should be set to 1458, it might seem to work with a higher setting, but some sites will break and you may see other intermittent problems.
I've not seen 1458 mentioned anywhere before.
Most people seem to say 1500/1500/1460 (MTU/MRU/MSS) for BT, but I'm happy to try 1458 - what about MRU/MSS? I guess 1458/1418 respectively?
NB: Most of the routers I've tried have had default settings for a BT connection and usually they're 1500/1500/1460 I think, I just noticed that the MSS looks out on this one, probably left over from one of the many tests I've tried recently.
<edit>I've now seen reference to 1458 as BT's preferred MTU so I'm trying it now.</edit>
When tweaking my router's MTU settings, should I need to change my PC settings at all? Presumably the router will fragment packets as required? I guess dropping the network down to 1458 would improve throughput, but it should work if I don't, yes?
BT (iirc) said some time ago they were going to fix the MTU "problem", so it could be that on the ATM backend it is working sometimes but when a different router comes into play for whatever reason on the ATM network that it breaks because you've got a higher mtu set and this could explain the behaviour you are seeing.
Just to be clear: if I ping with a packet size of 1472 and fragmentation disabled, I get consistent success at MTU=1500 (although having said that maybe I should recheck to be sure). If your suspicion is correct, presumably I'd be losing packets above 1458 (ie 1430 ICMP packet size)?