[c-nsp] Difference between Cisco Q-in-Q and IEEE 802.1ad

[sthaug at nethelp.no]

> I made a simple setup where Cisco Q-in-Q port("switchport mode
> dot1q-tunnel") was facing a laptop. Laptop was sending out frames with
> 802.1q tag(VID was 10) and Cisco Q-in-Q port pushed another 802.1q field.
> Final frame can be seen here:
> http://www.cloudshark.org/captures/ae485b68e9ed
> 
> Based on this Cisco Q-in-Q frame, the only difference between IEEE 802.1ad
> frame and Cisco Q-in-Q frame is that former has 0x88a8 as an TPID value for
> S-tag? I made a small illustration for this:

Correct, it's 0x88a8 vs 0x8100 for the S-tag.

> In addition, are there Cisco switches out there which use 0x88a8 as a TPID
> value of S-tag by default?

Don't know of any.

> Last but not least, which devices use 0x9100 as
> a TPID value for S-tags? According to drawing in Wikipedia IEEE 802.1ad
> article, some do:

Juniper E-series (ERX) routers use 0x9100 by default.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no