Long time no see ;-)
Straight on question now. Imagine a MPLS network with the following topology:
A B C D E
(X) --- (X) --- (X) --- (X) --- (X)
CE PE P PE CE
Router A & E are customer's routers.
Router B & D are PE routers
Let's say that we have created MPLS ATOM using Xconnect in between routers B and D. They are both using FastEthernet interfaces with sub-interfaces configured on. Router D is configured to RouterE in this way:
description ** RouterD->RouterE **
encapsulation dot1Q 15
no cdp enable
xconnect 18.104.22.168 666 encapsulation mpls
on the other end, router B is configured as follow:
description ** RouterB->RouterA **
encapsulation dot1Q 26
no cdp enable
xconnect 22.214.171.124 666 encapsulation mpls
Where 126.96.36.199 is RouterD loopback and 188.8.131.52 is Router B lo0.
What do you think about that scenario? Should it work with this configuration when the dot1q vlans differs? In my opinion this shouldn't work as expected as long as MPLS is doing just transparent transport of entire L2 frame (instead of using internetworking on IP level)
Can anyone, please explain how does Cisco handle this? I remember that I've read somewhere during my CCIE journey that there are different types of AtOM VC's which can either carry the dot1q tag or not.
Thank you in advance!
P.p. I tried it in a few different configurations and the results are very interesting but please first share your thoughts ;-)
You can't force the vc-type and don't need to.
- switchport trunk mode and subinterfaces will always pop the outer tag
- EVC interfaces do nothing by default.
On top of that vc-type 4 will add a service-delimiter tag to the frame received from the AC. It's the responsibility of the egress router to know what to do with this tag (rewrite or remove it).
GSR and 7200 will negotiate a vc-type 4 if the AC is a subinterface. 7600 will always negotiate a vc-type 5 except if the peer wants a vc-type 4.
With your configuration, PE's will negotiate vc-type 4 (VLAN mode). It means each transmitted frames must have a service-delimiter tag which is usually local significant. That's why this tag is re-written or removed at the egress PE to reflect its local configuration.
With VC-type 5 (port mode), the service-delimiter tag is removed.
Please check chapter 4.4.1 of RFC 4448