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

Brian Feeny signal at shreve.net
Wed Oct 27 08:43:38 EDT 2004

For those of you terminating DSL subscribers using PPPoE over an L2TP 
what MTU are you using for the L2TP tunnel between your LNS and the 

What other MTU adjustments are you making for this functionality?

We see problems in some situations with customers, and I am 
investigating this further.
If the customer is using a CPE, and doing NAT on that CPE (therefore 
traffic is being sourced
from the CPE), then all seems to work well.  Typically DSL CPE sets MTU 
to 1492 or works
thru PPP to negotiate proper MTU.  If the customer is using public 
routable IP's and is behind
the CPE, and is sourcing traffic from their workstation, then there are 
MTU problems.  The
PPPoE session is still between the CPE and the LAC, I am not talking 
PPPoE to the client

Adjusting MTU on customer PC's which may be any variant of OS, is not a 
practical solution.

The design of things looks something like this:

CPE <-----> DSLAM <-----> ATM Switch <-----------> BBG LAC <---------> 

Right now, the L2TP tunnel interface is MTU 1500.  I may have to 
experiment with
adjusting MTU there, as well as in PPP, but wanted to find out if 
anyone had
any insight on this since some of you are probably using this type of 


Brian Feeny, CCIE #8036, CISSP
Network Engineer
ShreveNet Inc.
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