[AusNOG] TPG EFM - low MTU

[Andrew Jones]

You're right, the PPP headers are still there, but not all 8 bytes of 
the PPPoE overhead.
The 40 bytes includes 4 bytes of PPP headers, so a 1540 byte MTU should 
be able to carry a PPPoL2TP service with an IP MTU of 1500 bytes.

http://www.cisco.com/c/en/us/support/docs/dial-access/virtual-private-dialup-network-vpdn/24320-l2tp-mtu-tuning.html



On 23.06.2014 15:43, McDonald Richards wrote:
> Pretty sure the PPP header is still there at that point but the 1492
> client wan normally fragments packets to prevent it being an issue so 
> 1540
> would still be your maximum required size to the LAC.
>
> Macca
>
>
> On 23/06/14 3:35 PM, "Andrew Jones" <aj at jonesy.com.au> wrote:
>
>>I don't think you need to add another 8 on top of the 40 for PPPoE,
>>because that part of the frame gets stripped before it goes into the
>>l2tp tunnel, unless I'm mistaken...
>>
>>On 23.06.2014 15:26, McDonald Richards wrote:
>>> You need to be able to hit a LAC at 1540 bytes from your LNS if you
>>> don't want to be fragmenting clients. Add another 8 bytes for 
>>> PPPOE…
>>>
>>> Macca
>>>
>>>  From: Bruce Forster <bruce at tubes.net.au>
>>>  Date: Monday, 23 June 2014 3:20 PM
>>>  To: "ausnog at ausnog.net" <ausnog at ausnog.net>
>>>  Subject: [AusNOG] TPG EFM - low MTU
>>>
>>> Hi Noggers,
>>>
>>>  I have a strange issue seems to be isolated to VIC LAC's
>>>
>>>  Some session are coming up at MTU 1476.
>>>
>>>  I did managed to clear a l2tp tun and the session came back via
>>> another LAC at 1500.
>>>
>>>  These are the ones i have had problems with: TPGVICLAC-6e
>>> / TPGVICLAC-5e
>>>
>>>  To get around it iv set mtu on the dialer to match and used a
>>> mss-adjust...
>>>
>>> I can ping to the LAC's IP at 1500 with no issue.
>>>
>>>  --
>>>  Regards,
>>>
>>>  Bruce
>>> _______________________________________________
>>> AusNOG mailing list
>>> AusNOG at lists.ausnog.net
>>> http://lists.ausnog.net/mailman/listinfo/ausnog
>>
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