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

[Brian Feeny]

Gert,

You are absolutely correct, and after later thinking about that I 
figured that out.  The incoming
interface to my LNS (incoming from the internet) reports back to the 
remote side when that next
hop can't handle the packet.

But still, the Link between myself and Bellsouth is only 1500, and 
since this is the link on which
l2tp tunneling is being done, thats going to create a problem with it 
being so low.  I did a test, and
my LNS reports back to remote sites MTU of 1492, which sounds correct 
for PMTU Discovery since
thats the PPPoE layer responding back its limit.  But PMTU Discovery 
has no idea about the L2TP
link, and so I have to take the MTU of that link and make sure it 
accounts for all overhead (1492 +
all l2tp overhead, etc).  I am going to set it for like 1560.  I am not 
sure but I think that would make
it all work.

Another "fix" that works, is to set "ip ignore-df bit" on the Interface 
on my LNS that terminates customers
PPP sessions.  This is not ideal of course, but gets things working at 
the expense of frag/reassembly performance
hit.

Brian

On Oct 29, 2004, at 6:14 AM, Gert Doering wrote:

> Hi,
>
> On Thu, Oct 28, 2004 at 09:03:15AM -0500, Brian Feeny wrote:
>> But in the Bellsouth BBG model, the L2TP tunnel is privately numbered
>> as well, so even if it could send an
>> ICMP for PMTU discovery purposes, it wouldn't make it far.
>
> The "outside" numbering of the L2TP tunnel has no influence on
> "inside" PMTU discovery.
>
> gert
> -- 
> USENET is *not* the non-clickable part of WWW!
>                                                            
> //www.muc.de/~gert/
> Gert Doering - Munich, Germany                             
> gert at greenie.muc.de
> fax: +49-89-35655025                        
> gert at net.informatik.tu-muenchen.de
>
---------------------------------------------
Brian Feeny, CCIE #8036, CISSP
Network Engineer
ShreveNet Inc.

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