[c-nsp] What MTU for Bellsouth BBG / BRAS <-> LNS l2TP tunnel?

Brian Feeny signal at shreve.net
Wed Oct 27 12:48:57 EDT 2004

So you are running MTU 4470 on the PVC over which your L2TP session is  
but I am using 1500, and thats probably the difference.

In the old days I use to think PPPoE was more ideal, because it could  
terminate on the
customer desktop (WinPoet, etc).  But of course DSL is almost always  
terminated into
CPE, so maybe I should look more closely at PPPoA vs. PPPoE.

I would still like to solve my MTU issues, which I probably should try  
to just raise the MTU
of my L2TP port.  PPPoA or PPPoE you still have a customer on ethernet  
somewhere negotiating
an MTU with a distance end, and I would think there is going to be alot  
of simularities in the problems
you find.


On Oct 27, 2004, at 7:59 AM, Jon Lewis wrote:

> On Wed, 27 Oct 2004, Brian Feeny wrote:
>> For those of you terminating DSL subscribers using PPPoE over an L2TP
>> tunnel, what MTU are you using for the L2TP tunnel between your LNS  
>> and
>> the BRAS/LAC?
> ATM2/0 is up, line protocol is up
>   Hardware is ENHANCED ATM PA
>   MTU 4470 bytes, sub MTU 4470, BW 40704 Kbit, DLY 190 usec,
> Funny thing though is we were never told it would do PPPoE.  We  
> migrated
> several ATM T1's of PPPoA customers to the BBG DS3, and left them all  
> as
> PPPoA.  I only recently found out by accident while testing new CPE
> hardware that the BBG will do PPPoE as well.
>> What other MTU adjustments are you making for this functionality?
> None.
> Perhaps you should look into doing PPPoA on the CPE instead of PPPoE.
> I've got an MTU of 1500 on the both the CPE side and the virtual-access
> interface on the server end and no MTU issues.  Since the PPPo[EA]  
> session
> is handled by Bell's shastas and sent to you as a L2TP session, you  
> don't
> even have to change anything on your end...just tell a CPE to do PPPoA
> instead of PPPoE and it should work.
> ----------------------------------------------------------------------
>  Jon Lewis                   |  I route
>  Senior Network Engineer     |  therefore you are
>  Atlantic Net                |
> _________ http://www.lewis.org/~jlewis/pgp for PGP public key_________
Brian Feeny, CCIE #8036, CISSP    	e: signal at shreve.net
Network Engineer           			p: 318.213.4709
ShreveNet Inc.             			f: 318.221.6612

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