Which statement is true regarding default MPLS label handling?
A. Arouter that popsalabel fromalabel stack always pushes a new label on the packet
B. Arouter that popsalabel fromalabel stack forwards a frame based on the label that was popped.
C. Arouter that popsalabel fromalabel stack forwards a frame based on the underlying destination IPaddress
D. Arouter that popsalabel fromalabel stack applies class of service based on the IP header to ToS bits.
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https://tools.ietf.org/html/rfc3031#section-3.16
I think that the answer should be B.
the answer depends on of the label is popped at the Penultimate hop or at the last or egress router… assuming the default behavior, then B is correct If pop operation is performed by egress then C is correct
C is the correct answer.
A cannot be the correct answer because of the same explanation you gave. It says “always pushes a new label on the packet”, and this will always happen if the LSR is not the egress LSR, nor the ingress LSR. However the question does not specify we are talking about an intermediate LSR.
B cannot be the correct answer since MPLS will never forward traffic based on a popped label. For a very basic MPLS scenario (which this is) this might not have sense. The most MPLS can do here is pop, push or swap labels but never take a routing decission based on the popped label.
D cannot be the correct answer since it simply does not make sense. MPLS will never automatically apply QoS policies.
C is the correct answer. At the very end, as you mentioned, the egress LSR will request the previous intermediate LSR to pop the label so that it (the egress LSR) can receive it without the need to pop that label and simply forward the packet; it will just forward based on the underlying IP address. 🙂