BT Business Hub 6 MTU

[#1]

I bought a brand new BT Business hub 6, it's been working fine, I'm on ADSL and it's mtu is 1492.

I tried setting it to 1472 but it won't change, even disconnected and saw mtu go to 0, so set it to 1472 and reconnected and the sob reset to 1492.

How can I get this darn thing to stay at the mtu I want ?

I used tcp optimizer and it tested and said to set my windows mtu to 1472...1444 + 28

[#2]

@Glenn 

Mine does the same despite it claiming to be saving the changes. A quick trawl on the net suggests that it can't be changed.

To add to the confusion, in my mind anyway, this article suggests that ADSL should use 1492 and PPPoE, which Plusnet uses for VDSL, 1480. I'm on VDSL and my Business Smarthub also defaults to 1492. I then read about extra bits that are used for this and that which need to be taken into consideration, which is more that my brain can cope with! https://support.microsoft.com/en-gb/help/314496/the-default-mtu-sizes-for-different-network-topologi...

Perhaps some-one more knowledgeable can explain it all to us?.

[#3]

@Baldrick1 

Thanks for the info, I was hoping you'd reply to the topic lol

In the hub's interface it leads you to think the mtu can be changed, after all why does it let you delete and input a different value, then it immediately wants you to save the change ?

So what do BT suggest people on ADSL do ?....just go with the mtu for fibre and tough luck ?

I have 2 pc's in a network connected to the business hub, if the hub is stuck on 1492 is there any point having my pc's set to 1472 ?

Extremely annoying that BT won't let us change it.

[#4]

As I understand it if the PC has an MTU of 1472, then the router will simply pass those 1472 byte packets out to the internet. It will only fragment packets if the PC sends transfer units larger than its own 1492 maximum.

So if you want traffic from the PC to the internet to have a maximum transfer unit size of 1472, you can set the PC to that value and things will (probably?) work as you want.

I have a Business hub with an MTU of 1492. I actually set up my PC's to have an MTU of 9000 (jumbo packets) because it does seem to improve the performance of file transfers on the Gigabit LAN and I don't notice any difference on my  FTTC internet link.

However I am actually now a BT customer, and all my traffic defaults to IPv6 where available - and I don't understand IPv6 to that level.

[#5]

Ah, so the MTU size works the same way as DNS server selection. That is, router settings are overridden by attached device settings. 

Thinking about it, that is why you can find the best MTU settings by trying different values using the ping command.

[#6]

Thanks for the info, on my Gigabit network my file transfer speeds were bad, it would start off fine then nose dive down to like 25MB/s, on the Realtek Nics in my pc's I disabled large send offload and that fixed it, get full gigabit speed now.

Interesting about Jumbo Frames, I was curious about them, read a few articles suggesting average ADSL users shouldn't use them.

[#7]

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@Baldrick1 wrote:

Ah, so the MTU size works the same way as DNS server selection. That is, router settings are overridden by attached device settings. 

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Almost - it's not either or however as with DNS. Its the MAXIMUM transfer unit. So if the PC maximum is smaller than the routers all is well. If it's bigger, then the router will fragment the packets as it forwards them.

[#8]

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@Glenn wrote:

read a few articles suggesting average ADSL users shouldn't use them.

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Average in the sense of connecting a PC or a couple of PC's to the internet - almost certainly not. But users who have NAS devices and other large scale data transfers on their network are probably not average - although increasingly common I suspect.