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Autores Olivier Bonaventure
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Packet handling by MPLS router

Consider the R7 MPLS router shown in the figure below.

https://inginious.info.ucl.ac.be/course/lingi2142-QCM/mpls-router/mpls-router.png

This router uses per-node label space and has the following LFIB

Label Outgoing intf Operation
L3 North-E SWAP( L0)
L4 East POP
L6 Local POP
L5 South-E PUSH (L1)
L9 South-East SWAP (L8); PUSH (L4)

For this exercice, the MPLS stack of a packet is represented as as sequence of labels separated by a colon. For example, LX:LY:LZ is an MPLS stack containing three labels. LZ is the top label, LY the second one and LX the bottom one.

Questão 1: Packet observed

We observe a packet with the L2:L0 label stack on the link to R1. What is the packet that router R7 has received ?

Questão 2: Packet observed

We observe a packet with the L8:L4 label stack on the link to R3. What is the packet that router R7 has received ?

Questão 3: Packet observed

We observe a packet with the L0 label stack on the link to R1. What is the packet that router R7 has received ?

Questão 4: Packet observed

We observe a packet with the L0:L5:L1 label stack on the link to R3. What is the packet that router R7 has received ?

Questão 5: Packet observed

We observe a packet with the L5 label stack on the link to R2. What is the packet that router R7 has received ?