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What's going on ?
This is a discussion on What's going on ? within the Sky Broadband help forums, part of the Sky Broadband help and support category; My MTU is set to 1130 and I still get the odd disconnections every so often....
- 29-10-06, 08:26 PM #11
My MTU is set to 1130 and I still get the odd disconnections every so often.
- 29-10-06, 09:16 PM #12
This is the post I was looking for, as quoted from Sky Digi Forum:-
'I'm sceptical. The MTU (maximum transmission unit) simply sets the maximum packet size to be transmitted.
If you try to use a MTU that's too large for the network then the packets get fragmented - for a PPPoA connection, whilst obviously that's not a good thing it's not a disaster. It just means you're transmitting more packets than is needed so the impact is on speed. With PPPoE there will be more significant negative effects.
You can test your network MTU with a simple Windows ping (e.g. ping -f -l n bbc.co.uk) command. If you do this then you'll find that the maximum packet size (n) when on Sky LLU is 1472. To this you need to add 28 bytes for the header giving you a MTU of 1500. i.e. there is no reported fragmentation with an MTU of 1500.
I've seen several posts recently where Sky are advising a reduction in the MTU. Interestingly I've never seen the same figure quoted twice! The idea of subtle changes of MTU (from 1500 to 1450 having a visible and significant difference when accessing say bbc.co.uk is, in my view, rather suspect.
Even if there is something odd happening in the Sky network that requires a reduction (and if there is then it's invisible to the ping test) then for it to have an impact on connections (rather than speed) then it is completely breaking the protocols which govern transmission. This doesn't seem likely.
For the BT network (and presumably Sky Connect?) a MTU of 1458 has been long recommended. This stems from some technical implementation in the early days of ADSL. I thought this was due to have been resolved back in 2003 when BT made some infrastructure changes but I've no idea whether this was done or not.
So, dropping your MTU to 1450 (or any other similar figure Sky seem to make up) shouldn't have any really significant negative impact (maybe a <5% speed reduction) if someone wants to follow this advice then it will do no real harm. If having a MTU of 1500 (the router default) could cause a total loss of connection then logically every customer would have this problem.
As a test for my own interest I dropped my MTU down to 1450. Not surprisingly I couldn't see a difference in browsing sites like the BBC. Testing the download of a large file over FTP from a reliable site I could see a small speed drop. I'm back on 1500. YMMV.'
- 29-10-06, 10:08 PM #13
I was told earlier today to set mine to 1440 as my Sky router was constantly locking up, also told to tick automatically detect settings in the lan box of internet options well did this and 5 mins after saying goodbye router locked again back on my linksys router since then and no problems upto now