From: robbie (robbie@packetized.org)
Date: Thu Jun 24 2004 - 09:05:21 GMT-3
Yes, however if you're running PPPoE (let's say with a Cisco PIX 501
doing PPPoE auth via a DSL bridge), you're effectively encapsulating an
ethernet frame inside of a PPP packet, correct (layer 2 upconversion to
layer 3, much as what happens in L2VPN/MPLS)? If you start running
larger frames close to the default MTU of the PIX (1500, right?) I've
found that some protocols will break - most recently pages like Hotmail,
Yahoo, etc, simply because your MTU is too large to shoehorn some of
these large ethernet frames onto the PPPoE segment. 1500 bytes + PPP
overhead, being shot into another 'ethernet' space causes some stuff to
break. Lowering the MTU on the PPPoE CPE termination device to 1492
bytes resolved this problem, so I assume my hypothesis is correct.
In the PPPoA world, you don't have to worry about the additional
overhead, because the AAL5 MTU is so much larger - but I imagine you
would be hard pressed to find a CPE device aside from a DSL router
(Zyxel and Cisco is what we use) that can temrinate a PPPoA session,
mostly because most DSL bridges don't output ATM on the client interface.
Does this make any sense? I'm trying to just make certain that I'm not
completely off my rocker. :)
Robbie.
Scott Morris wrote:
> You CAN run PPPoE as well, at that point, you're extending the PPP session
> out to the CPE device, not just the DSL modem. So it just depends on where
> you are running the code at. Some providers to PPPoA (transparent to users)
> others use PPPoE clients that the user must configure.
>
> Either way, they're doing something to separate traffic a bit more than
> previously done. PPPoE can also be run in cable modem space (some MSOs
> are).
>
> The MTU is based on whatever you're transiting, minus some extra overhead.
> In PPPoA, why would you care what the MTU is there? You're still attaching
> to the network with your computer on an ethernet segment, so you'll still be
> in a 1500 byte world for the most part.
>
> But there's lots of different implementations depending on what boxes you're
> using and how bright the engineers happen to be! And user support as
> well...
>
> One o' them things...
>
>
> Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713, CISSP,
> JNCIP, et al.
> IPExpert CCIE Program Manager
> IPExpert Sr. Technical Instructor
> swm@emanon.com/smorris@ipexpert.net
> http://www.ipexpert.net
>
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> robbie
> Sent: Wednesday, June 23, 2004 10:30 PM
> To: ccielab@groupstudy.com
> Subject: Re: Question on ATM...
>
> As an aside, I recently had a top 5 firewall vendor tell one of me and one
> of my co-workers that all DSL is PPPoE, including but not limited to HDSL,
> SDSL, ADSL, etc. Considering that my employer is an ISP offering DSL, and
> using PPPoA and not PPPoE, I had quite a laugh at his expense.
> From what I understood, PPPoA allows one to use use a larger MTU (4352) all
> the way to the CPE device, instead of 1500 to the CPE device and
> 1492 for any devices behind said CPE device. There are more reasons that
> seem to elude me at the moment - can anyone else expound on this?
>
> Robbie
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