The MPLS WG Archive

[Shen Gangxiang <EGXShen@ntu.edu.sg>]

Sudhanshu Jain wrote:
> 
> L3PID is the what used in setting up tunnel which are protocol aware.
> 
> But if a tunnel is being setup as a fat pipe, ( e.g. L3PID is not
> present at setup time).
> 
> Is it possible for such a tunnel to carry multiprotocols via using 0
> or 2 label at the bottom of the stack?
> 
> Is there any existing implementation which carry multi protocol over
> single tunnel? If so, how do they carry protocol information
> end-to-end?
David worte:

Doing this is very problematic.

Eventually, the data will have to leave the MPLS tunnel.  When it does,
some kind of L3PID information must exist, in order to place it in the
L2 header of the network beyond the tunnel.  If multiple packets in the
tunnel have different L3PIDs, how will the egress router be able to
properly generate the L2 header?

As pointed out by the draft, "the identity of the network layer protocol
   must be inferable from the value of the label which is popped from
   the bottom of the stack, possibly along with the contents of the
   network layer header itself", so based on this, we may recognize
the upper layer protocols. As to how to generate the L2 header, I think
it is up to if the egress router can support such a kind of function.

Gangxiang