[Cerowrt-devel] Baby jumbo frames support?

Dave Taht dave.taht at gmail.com
Wed Jun 20 20:58:45 EDT 2012

On Sat, Jun 16, 2012 at 5:53 AM, Alexander Hulse <ah at amch.net> wrote:
> I've done some basic research and some (unsuccessful) hacking the on the driver source for the ag71xx driver and was wondering about baby jumbo frame support?
> I know that the WNDR37/800's built in switch supports passing jumbo frames, but the underlying router interfaces do not support large MTU frames.
> However, this:
> http://wiki.mikrotik.com/wiki/Manual:Maximum_Transmission_Unit_on_RouterBoards
> seems to imply that the built-in interfaces ought to support baby jumbo frames, unless I'm misunderstanding what they are saying.
> As to the "Why?", in the UK, the fibre offering by BT using FTTC uses a PPPoE modem that can understand PPPoE packets with an MTU 1508, as described in RFC 4638.
> It was possible using a Netgear WNR834T with the application of some patches from the pppd git:
> http://git.ozlabs.org/?p=ppp.git;a=commit;h=fd1dcdf758418f040da3ed801ab001b5e46854e7
> http://wiki.aa.net.uk/index.php/FTTC_Modem
> Would this be a useful feature to add (if possible) to CeroWrt so that full sized frames can be used if the ISP / connection supports it?

I don't quite understand what you want here.

The internal ethernet connections on the netgear 3800 are both
connected to the switch. The switch is capable of jumbo packets, the
ar71xx driver appears to have vlan support only. I'm told it does not
have jumbo packet capability, but as to what it's upper limit is, is

As for PPoE with a size 1508... um... one or the other device is going
to get in your way here. I presume that 1500 works? You would do
better to contact the author of the driver (juhosg) to get your
question answered as I'm under the impression he is under the right

I note that in cerowrt the switch has been configured for jumbo frames
even tho the ethernet chip can't do that - because it cuts latency
down by a lot to pre-allocate for jumbo frames as the on board
buffering on the switch is quite large - in fact, I regard it as too
large, even now...

> Alex
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> Cerowrt-devel at lists.bufferbloat.net
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Dave Täht
http://www.bufferbloat.net/projects/cerowrt/wiki - "3.3.8-6 is out
with fq_codel!"

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