[c-nsp] q-in-q mtu

[Antonio Querubin]

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