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

[sthaug at nethelp.no]

> I think 802.1ad defines 0x88a8 as the prefered outer tag (different
> from 0x8100 of 802.1q), which makes it much easier for switches to
> know whether this is a single or a double-tagged frame.  And i think
> Extreme & Juniper follow this.

Extreme uses 0x88a8 as the default outer Ethertype, but newer switches
let you configure any value, including 0x8100.

Juniper uses 0x8100 as the default outer Ethertype.

> On the other hand, Cisco uses what i would call a "hack", since it
> seems to support double-tagging by just using the single-tagging
> code (checking if the ethertype is 8100).

The Cisco solution may be a hack, but it's a very *useful* hack. Given
our operational experience, we would probably not use 0x88a8 for the
outer Ethertype even if all equipment supported it - it's simply not
flexible enough.

It *might* be useful to have a different Ethertype than 0x8100 for the
outer tag if you could define this on a per-port basis. For all switches
we've looked at so far, however, it is defined on a per-switch basis.

> IMHO, the best solution would be to be able to configure both
> inner/outer ethertypes manually.

I agree that it is useful to be able to configure the outer Ethertype.
I don't see the use for configuring the inner Ethertype.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no