Where
does this label stack reside? The label stack sits in front of the Layer 3
packet—that is,
before
the header of the transported protocol, but after the Layer 2 header. Often,
the MPLS label
stack is called
the shim header because of its placement.
Q: How we
can know if the next address is L2 Header or MPLS Label ?
the label stack is present after the PPP
the label stack is present after the PPP
header
but before the IPv4 header. Because the label stack in the Layer 2 frame is
placed before
the
Layer 3 header or other transported protocol, you must have new values for
the Data Link
Layer Protocol field, indicating that what follows the
Layer 2 header is an MPLS labeled packet.
The Data Link Layer Protocol field is a value indicating
what payload type the Layer 2 frame is
carrying
Example :
R1 loopback try to ping R3 loopback using R1# ping 3.3.3.3 source loopback 0
R1 loopback try to ping R3 loopback using R1# ping 3.3.3.3 source loopback 0
Topology and
advertised labels as shown :
Result : there’s new filed in
L2 Header called type that to notify the router the next header will be MPLS
stack label using this Hex number as shown in above in table and below packet
capture.
Note : Where does MPLS fit in? MPLS is
not a Layer 2 protocol because the Layer 2 encapsulation is
still present with labeled packets.
MPLS also is not really a Layer 3 protocol because the Layer 3
protocol is still present, too.
Therefore, MPLS does not fit in the OSI layering too well. Perhaps
the easiest thing to do is to view MPLS as the 2.5 layer
and be done with it.
