The MPLS WG Archive

[David Charlap <David.Charlap@marconi.com>]

Sandeep B wrote:
> 
> You mention using 0x0800 as L3PID no matter what intermediate L2
> protocol is carrying this traffic. Lets say we have a LDP session
> running over another RSVP Session and this session is a VPN for a
> frame relay circuit carrying IP traffic. Wouldn't we use a L3PID of
> LDP-MPLS (0x8847) instead of IP for the RSVP session.

No.  When you tunnel one LSP through another, you should push a new
label onto the label stack.  The labeled packet's payload is still IP.

You do not send the entire labeled packet - headers and all - into the
inner LSP.

For example, given this topology:

             /\       /\            /\       /\
	____/ A\_____/ B\__________/ C\_____/ D\____
         1  \  /  2  \  /     3    \  /  4  \  /  5
             \/       \/            \/       \/

Router A is an ingress router for the outer LSP.
Router B is an ingress router for the inner LSP.
Router C is an egress router for the inner LSP.
Router D is an egress router for the outer LSP.

An unlabeled IP packet arrives at router A on segment 1.

The packet that is sent from A to B on segment 2 should be a labeled
packet with one label.  Assuming an interface that uses generic
encapsulation, it will look like:

	SHIM HEADER (with one label)
	IP packet

The packet that is sent from B to C on segment three should be a labeled
packet with two labels.  It should look like:

	SHIM HEADER (with two labels)
	IP packet

Please note that the payload under the shim header is still the IP
packet.  Which is why the L3PID should still be 0x0800.

It is not what you were describing:

	SHIM HEADER (with one label)
	SHIM HEADER (with one label)
	IP packet

In this configuration, the payload of the first shim header would be
another shim header - making the L3PID for the inner LSP 0x8847.  But
this is not how hierarchical LSPs are supposed to be established.

-- David