Changing mtu to 1508

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choppyc
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Joined: 31 Mar 2018
Posts: 4

PostPosted: Sat Mar 31, 2018 20:19    Post subject: Changing mtu to 1508 Reply with quote
Hi, i cannot change on web gui so I’ve ssh into router and ran ifconfig ppp0 mtu 1508, I then lose internet connectivity, could some please help. Running r7800 with Kong build. R35550M
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Per Yngve Berg
DD-WRT Guru


Joined: 13 Aug 2013
Posts: 5635
Location: Akershus, Norway

PostPosted: Sun Apr 01, 2018 6:02    Post subject: Reply with quote
1500 is the max. Ethernet frame. With the pppoe overhead you will have less.
choppyc
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Joined: 31 Mar 2018
Posts: 4

PostPosted: Sun Apr 01, 2018 8:30    Post subject: Reply with quote
Most ISPs in the UK support something called a baby jumbo frame. They have adopted the standard RFC4638 which allows for an MTU bigger than the normal maximum of 1500 bytes, specifically they've allowed for a 1508 byte MTU.

This means that your router can use a 1500 byte MTU for your connection and still have space for 8 bytes for the PPP session which takes you to the 1508 baby jumbo frame.

But I’m unable to change this, any ideas? My modem supports mtu 1520, it has worked using OpenWrt .
Alozaros
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Joined: 16 Nov 2015
Posts: 3655
Location: UK, London, just across the river..

PostPosted: Mon Apr 02, 2018 6:41    Post subject: Reply with quote
yep 1508MTU is modem supported, but not dd-wrt supported,
i use BT and Plus and they stick to 1492 MTU for pppoe
not much point to use it as many devices do not support more over 1500 MTU...

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Mevunky
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Joined: 06 May 2013
Posts: 24

PostPosted: Sat May 05, 2018 0:19    Post subject: Reply with quote
I can't seem to get this working from the shell by changing the MTU's of the required interfaces either.

Anyone had any luck?

Possible to get this feature added to DD-WRT as PPPoE with 1508 support is becoming common.
Per Yngve Berg
DD-WRT Guru


Joined: 13 Aug 2013
Posts: 5635
Location: Akershus, Norway

PostPosted: Sat May 05, 2018 6:40    Post subject: Re: Changing mtu to 1508 Reply with quote
choppyc wrote:
ifconfig ppp0 mtu 1508


Wrong interface. It's the interface that PPPoE is running over that have to be increased. For an R7800 that is eth0.

ifconfig eth0 mtu 1508
Mevunky
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Joined: 06 May 2013
Posts: 24

PostPosted: Sat May 05, 2018 9:29    Post subject: Re: Changing mtu to 1508 Reply with quote
Per Yngve Berg wrote:
choppyc wrote:
ifconfig ppp0 mtu 1508


Wrong interface. It's the interface that PPPoE is running over that have to be increased. For an R7800 that is eth0.

ifconfig eth0 mtu 1508


Tried that, increasing eth0, then vlan2 to 1508 and lastly pppoe to 1500 but it drops and then resets the connection/mtu settings.
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