[c-nsp] q-in-q mtu

[Daniel Lacey]

 Isn't 1490 the "magic" MTU for PPPoE?

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On 10/21/10 11:15 AM, Antonio Querubin wrote:
> On Thu, 21 Oct 2010, David Freedman wrote:
>
>> Problem is the DEC chip found on PA-FE-TX , it has a hard limit of 1530
>
> It's actually the built-in FastEthernet port instead of a port adapter:
>
> #show controllers fastEthernet 0/0
> Interface FastEthernet0/0
> Hardware is DEC21140A
>
>> PDU (i.e on the wire, with headers), IOS prevented you changing the mtu
>> on the interface (which is usually the SDU, i.e without the ethernet
>> frame overhead) because it was so close to the edge, but with advent of
>> "tag-switching mtu" you were allowed to "bleed" into this space,
>> in later releases they abandoned tag-switching mtu in favour of just
>> letting the user configure the mtu directly up to the 1530 limit (but
>> woe betide you if you configured it 1530 as this is the assumed SDU and
>> hence had a PDU >1530!)
>
> This IOS is either too old or too crippled.  It doesn't have
> 'tag-switching mtu' and the max configurable mtu is 1500.  So I'm
> still wondering why (according to the carrier doing the testing):
>
>  - Setting our sub-interface IPv4 mtu to 1490 works but 1492 does not
>
>  - IPv6 isn't affected
>
> Antonio Querubin
> 808-545-5282 x3003
> e-mail/xmpp:  tony at lava.net
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