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

Jon Lewis jlewis at lewis.org
Wed Oct 27 08:59:28 EDT 2004

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?


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                |
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