[j-nsp] MX5-T VPLS fowarding problem

[sthaug at nethelp.no]

> I got an off-list message from Diogo saying that the logical interface 
> (VLAN ID) on each side must be the same, unless you do some 
> pop/push/swap magic. Changing that did solve the problem! I still don't 
> see why though. I only use the VLAN locally on each site to separate the 
> traffic between multiple customers in the access switches, so the VLAN 
> tag should never be included in the actual frames forwarded between the 
> routers.

The BGP-based VPLS RFC (RFC 4761) specifies in section 4.1:

   Ethernet frames received from CE devices are encapsulated for
   transmission over the packet switched network connecting the PEs.
   The encapsulation is as in [7].

where reference 7 is RFC 4448, which in its turn says:

   In Ethernet PW operates in one of two modes: "raw mode" or "tagged
   mode".  In tagged mode, each frame MUST contain at least one 802.1Q
   [802.1Q] VLAN tag, and the tag value is meaningful to the NSPs at the
   two PW termination points.

Tagged mode is what you get if you have a VLAN subinterface and you
don't do anything specific to remove the tag. Thus you should *expect*
the VLAN tag to be included.

It would have been possible for Juniper to automatically translate
VLAN IDs on output - this is what for instance Cisco does on single-
tagged pseudowires. Such automatic translation means that the VLAN
IDs don't have to match. However, Juniper has chosen not to do such
automatic translation.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no