[c-nsp] IEEE 802.1ad vs Cisco's Q-in-Q

[sthaug at nethelp.no]

> Does anybody know when (and if) is Cisco going to support the offical
> 802.1ad?

No idea.

> If i understand correctly, Cisco's Q-in-Q implementation doesn't
> support different ethertypes for inner & outer tags, which probably
> means that Cisco switches don't care whether a frame is single or
> double-tagged, as long as the outer ethertype is 0x8100 (or equal to
> the one configured).

Why would you *want* to check this? Are you sure other vendors check?

We have a significant installed base of Extreme switches using a non
8100 Ethertype for the outer tag. It's a royal PITA to use, and we're
moving all new installations to simply using 8100 for outer and inner
Ethertypes. Much more flexible, saves us lots of headaches. We have
assumed from the start that the switches only look at the Ethertype
of the outer tag, and based our network architecture on that.

> So, are there any performance limitations when a double-tagged frame
> passes through a 6500 (or other switch)? What happens if the inside of
> every frame has to be checked (because of QoS) and a double-tagged
> frame is found? Does the switch send the double-tagged frame to the
> cpu because it cannot recognise the protocol inside it?  -- Tassos

As far as I know the outer tag is checked, and that's it. Nothing from
the inner tag is checked. No performance issues with double tagged
frames, because the switches simply inspect the outer tag and forward
based on that.

If you need equipment which can look at both outer and inner tag, Cisco
will be happy to sell you GSR linecards which can do it, Juniper has the
IQ PICs (which we use heavily), etc. You might be able to find high end
switches which can do something sensible based on inspection of both
outer and inner tags - but don't expect "normal" switches to do it.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no