[ILUG] Linux Router + DSL

[Thomas Bridge]

On Mon, Oct 18, 2004 at 03:54:10PM +0100, Paul Jakma wrote:

> On Mon, 18 Oct 2004, Thomas Bridge wrote:
 
> >On Fri, Oct 15, 2004 at 12:34:08AM +0100, Paul Jakma wrote:
 
> >>1. NB: Would it kill the ADSL providers to bump up the MTUs on their
> >>link with Eircom (for eircom wholesale provision) or the ATM VC with
> >>the customer by 8 bytes so the encapsulared PPP link doesnt have to
> >>suffer 1492 MTU?

> >Well yes.

> >You see we use PPP over Ethernet - Ethernet has an MTU of 1500, but
 
> s/has/once had/

For the purpose of this conversation there's no difference.   Eircom
can make no assumptions about vendors (and although Cisco will happily
accept MTU of 1500 for PPP over L2TP, when it's PPPoE they insist on
1492).

Also, far more importantly, we had a serious issue when we first resold
L2TP products with cheap ass CPE equipment that didn't like the fact
we were looking for a higher MTU.   Hence why it's sent
to 1492.

> >the PPP bit takes 8 bytes, hence 1492.
 
> Really? ;)
 
> How much of this goes over actual ethernet links though? The ATM VC 
> to eircom - doesnt care about ethernet MTU... the circuit between 
> Eircom and you, surely can be adjusted.. That leaves the internals of 

Listen, the amount of pain I've had with Eircom and MTU sizes in the
last month would keep a dominatrix in business for weeks.   Get me drunk
when I leave (this Friday) and I'll tell you all.

> Eircom's router^W"BAS" - unless they terminate the VC on a different 
> machine to where they encapsulate it into L2TP, that leaves only the 
> link between the BAS and the router handling the link to the ISP.

Where the assumption it's a direct link from the Eircom BAS to the OLO BAS?

Also, the fact that Eircom's network may support jumbo frames doesn't mean
the OLO's BAS will.   And Eircom in fairness have to support the LCD.
 
> However, in my case, the ATM VC terminates directly with you, doesnt 
> it? And /you/ can just bump up the MTU on your router, no need for 
> eircom to be involved.

Actually, IOS is setting the MTU, not an admin thing.   And I'm not
hiking up the configuration on the BAS just to suit you - especially in
my last week :)

However, as we're not hardcoding the MTU in this case, play with it
and see what happens.

As always in this situation, if it breaks, you get to keep both
pieces and support will be out of glue when you call!

Thomas