MTU Problem virgin media.

[sambache3592]

Hi Guys. I have the 20mb broadband connection with virgin media, anyway i bought an xbox yesterday and i am wishing to sign up to live. So i have managed to set it up so that it passes the ip test etc but it cant get past the MTU, i hav done some tests and it seems the MTU is restricted at 1300? I go into the wireless router and check, that says 1500, try lowering it a bit, still no effect. So I am wondering if this could be something to do with virgin media?

I want to know if it could be them before i ring them up, any helps much appreciated, and any other information you need give us a shout.

Just to let you know the wireless router is a Dynamode 54mbps, BR6004WG1.
And i am running windows xp.

Cheers:)

[nopcode]

All I can find regarding the MTU issue is to set the MTU to either 1364 or 1365.

I have never had to play with that on the router or the 360.

Maybe some other pc or lappy on your home network downloading alot of stuff and degrading your xbox traffic?


If the two MTU's work then please do tell, incase I get this problem in the future.

[sambache3592]

Have not other computers on the network, just need to find out what is causing the mtu to be restricted to 1300...

[nffc]

Mine is 1500 and works fine.

[foddy]

What's the problem with the MTU set to 1300, is it not working, or do you simply want a higher MTU?

The MTU is valid for the connection between two devices - the standard for ethernet is 1500. But, the MTU on any particular path between you and the server may be less than this. In particular, with ADSL and VPNs, the MTU on the link is reduced by a few bytes because of the additional header.

Because you don't know the lowest MTU between you and a server, there are two options - either the packets are fragmented before being sent over the low MTU network, or path MTU discovery is used. Fragmenting can cause performance issues, which is why PMTU discovery is the default on most systems.

PMTU discover works by setting the "don't fragment" flag on the packets. Instead of fragmenting it, an ICMP "would fragment" packet is sent back. The sender then tries with a lower MTU until the packet gets through - it has then learned the maximum MTU it can use on that path.

The problem is that some people (and device manufacturers) seem to believe that blocking ICMP packets is a good idea. It's crazy, as they're needed for the Internet to work ... if you rely on PMTU discovery and have ICMP blocked, TCP connections will hang, known as a PMTU black-hole.

Now, if this is the case, the only option is to reduce your MTU so that it works for most of the net (although it won't work universally, because MTUs can be much lower than 1300).


I've not got an XBOX, so I've no idea what it does - but I suspect either it sets it to 1300 to be safe, or it has detected that something on the path between you and Microsoft blocks ICMP, so has reduced its MTU to cope with it.

If everything is working, you shouldn't have any problem using a MTU of 1300. The difference in performance is minimal.

[sambache3592]

In order to use xbox live you must have an MTU of 1365 or above, at the moment i can only get a maximum of 1300 on the MTU, so therefore cannot connect to xbox live.

I have tried putting the xbox directly into the calbe modem, this works but isnt a full solution as i need the internet upstairs as well as the xbox.

So basically it is either the xbox or the computer, not virgin media...

[maverick]

For your information the MTU for a Virgin media cable modem is set to 1500 and cannot be changed.This is not a virgin media issue this is confirmed by the fact all works fine when connecting direct to the cable modem and not using your router.

[smudger2001]

Hi Guys. I have the 20mb broadband connection with virgin media, anyway i bought an xbox yesterday and i am wishing to sign up to live. So i have managed to set it up so that it passes the ip test etc but it cant get past the MTU, i hav done some tests and it seems the MTU is restricted at 1300? I go into the wireless router and check, that says 1500, try lowering it a bit, still no effect. So I am wondering if this could be something to do with virgin media?

I want to know if it could be them before i ring them up, any helps much appreciated, and any other information you need give us a shout.

Just to let you know the wireless router is a Dynamode 54mbps, BR6004WG1.
And i am running windows xp.

Cheers:)
i just come across this site after endlessly searching for an answer to my problem though it has nothing to do with mtu's it has to do with the fact you have the same type of router as me and you seem to have passed the network test with your xbox mine does not!
the router is set to dhcp and happily assigns ip's out to all and sunder but not the xbox360 (in the network setup page on the xbox ip, default getwaye etc are set to automatic yet they all read 0.0.0.0
did you do anything special to get it all up and running?

[The PIT]

Have you tried setting mtu on the router.

http://www.3-c.coop/support/upgrademtu.html

[Ignitionnet]

Holy thread necromancy with a post totally unrelated to the topic Batman.

[The PIT]

Holy thread necromancy with a post totally unrelated to the topic Batman.

Missed that doh it already claims to be 1500

However next step would be resetting the router too defaults and see what happens.

Also would be worth if there's a pc handy doing some ping tests with a packet size.

ping www.bbc.co.uk -l 1470

If the mtu size is 1500 you shouldn't get any losses.

Using wireless of course.

[Ignitionnet]

Nearly, but you forgot a couple of things. You forgot to set 'do not fragment' bit and got the size of the ping slightly low, 1500 MTU will carry a 1472 byte payload in the ping.

The command would be: ping www.bbc.co.uk -f -l 1472

[The PIT]

Nearly, but you forgot a couple of things. You forgot to set 'do not fragment' bit and got the size of the ping slightly low, 1500 MTU will carry a 1472 byte payload in the ping.

The command would be: ping www.bbc.co.uk -f -l 1472

Doesn't really matter we're just trying to find the actual mtu size. If it says the packet needs to be fragmented it tells you that you already reached the limit.

If the router is set at 1300 there will be 100% loss anyway.