MTU

[LeeM87]

I have the EE home router 2, model number b625-261 And the MTU is lower than the my Xbox series s would like. How can I increase the MTU, I'm struggling to find the setting. Thanks

[Captgeneralmark]

@LeeM87 On my Brightbox 2, the MTU settings are on the advanced/broadband settings page. The maximum value allowed is 1492, Which is what mine is set to.

[XRaySpeX]

You're unlikely to find an MTU setting on an EE mobile router. They are dumbed-down cousins of real routers.

Have you looked amongst its WAN settings?

[mikeliuk]

In general, do not change the MTU on any network interface or vlan (i.e. on switch) which must send and receive packets from the internet as the default of MTU 1500 would be the most appropriate setting.

I wouldn't care about a few bytes off that which may be to account for various packet headers. If the default lower value is correct and you increase it, you may get packet fragmentation and worse performance, or see some traffic dropped entirely.

For internal vlans and interfaces which never need to send or receive packets from the internet, MTU 9000 on switches is fine but you might want interfaces with slightly lower MTUs to account for headers (e.g. TCP, vxlan, vlan).

Edit: your best bet is to lower the MTU on you device to match the service provider's switch/router as the default value is likely to be correct and will work and any higher value could cause a hard to debug problem.

[mikeliuk]

Hi @Captgeneralmark ,

A search online for sizes of TCP, vlan, and vxlan headers shows that 8 bytes is the size of a vxlan header so that default value has likely been chosen to allow the use of vxlan (or similar) on the service provider's network. 😃

[XRaySpeX]

The MTU size for BTw-based BB is 1492. Dunno what it is for mobile networks.

[LeeM87]

Mine is 1320 and the Xbox needs 1380 I believe. 

[LeeM87]

I don't seem to have a WAN settings. Will EE likely change the MTU for me? It's there anything I can do hardware or software to change this?

[mikeliuk]

The MTU of interfaces on devices should be less than or equal to the value on routers and switches.

I can't think of any good reason why a service provider would supply a router misconfigured but 1320 feels lower than might be reasonably justifiable. I see some online references suggesting the Linux tracepath command can detect MTU changes on a route but it's probably better to wait for someone who knows how high the router MTU could be set to.

If you wanted to experiment, you could set the router to 1492 and tracepath to some known destination and lower the router MTU to the lowest MTU found on the route. Packets which are less than the MTU will be transmitted just fine so you're unlikely to break anything.

Having just done a test, MTU 1450 (Correction: actually full MTU 1500 as below) traffic passes for 4G EE SIM in a Netgear M1.

This is based on Red Hat documentation https://access.redhat.com/solutions/2440411

ping -M do -s 1422 domain.com # MTU is 28 bytes higher

Edit: it's disappointing you don't have the option to change that as I get the feeling someone slightly incompetent took a guess and has negatively impacted a lot of people with no current way to fix.

Edit2: correction, I can get a full MTU 1500 ping to pass. The previously mentioned limit was due to the target.

ping -M do -s 1472 domain.com # passes

[XRaySpeX]

For Windows you would use:

ping -n 1 -f -l size bbc.co.uk

If size was too big it would respond:

"Packet needs to be fragmented but DF set.".

You would use a loop counting down to discover the largest packet size that didn't fragment, viz:

for /L %i in (1472,-1,1440) DO ping -n 1 -f -l %i bbc.co.uk

The 1st one that doesn't "Packet needs to be fragmented" is the largest packet size wanted (For BTw-based BB it's 1464). 

Then add 28 to get the MTU for the network (For BTw-based BB it's 1492).