Re: [LARTC] Redundant internet connections.

From: Peter Rabbitson <rabbit@rabbit.us>
To: lartc@vger.kernel.org
Subject: Re: [LARTC] Redundant internet connections.
Date: Thu, 21 Jun 2007 17:37:52 +0000	[thread overview]
Message-ID: <467AB770.8080500@rabbit.us> (raw)
In-Reply-To: <467A2354.1070805@riverviewtech.net>

Grant Taylor wrote:
> No, again, if you are dealing with modem router combos, I'll grant you 
> what you say, but not on bridging modems.

*nod* I had several cases when my ISP had problems like the one ou 
describe below, so the first 2 hops were pingable but nothing outside.l 
This is why I suggested the entire ISP subnet exclusion, just to be on 
the safe side.

>> This would be a problem with your router configuration. It is 
>> virtually impossible to have an upstream problem that would cause 
>> this. It either works both ways or does not at all.
> 
> No, it was not a fault with my router.  It was a fault radio in an 
> (W)ISPs core network.  Completely out of my control.  When the ISP 
> replaced the piece of equipment in their core (not even on the link to 
> me) things started working correctly again.

I got to give you this one. Murphy at work.

> *nod*  I am presently using dual load balanced SDSL circuits with 
> automated (OSPF) failover at my office.  This is working out VERY well. 
>  However the questions I'm asking have to do with a project for a 
> different client.

No contest here either. It's just rather rare for a small scale end-user 
to be able to get access to IGPs.

> asymmetric routes.  Also, seeing as how both circuits are an ethernet 
> connection that can carry a frame size / MTU of 1500 byes, I don't see 
> the problems that would be introduced by encapsulated traffic like PPPoE 
> for one link verses the other link.  In short, I'm willing to listen to 
> problems with the asymmetric routes, but I have yet to hear any thing 
> that concerns me or even chafes me a little.
> 

I misread the part about the stuff behind the router being routable. 
There is nothing wrong with asymmetric routing in this case. However you 
bring up an interesting point about MTU, only to dismiss it right there. 
I think you will have a problem with the default MTU of 1500 being 
combined with the effective MTU of PPPoE links being 1492. Too many 
systems in this day and age have PMTU discovery enabled, and you know 
what is the current state of ICMP messaging on the net.

Peter

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