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Author(s) 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-2/mpls-router.png

This router uses per interface label space and has the following LFIB

Incoming intf Label Outgoing intf Operation
East L1 North-E SWAP( L7)
East L2 East POP
East L3 North-E POP
East L4 South-E PUSH (L2)
West L5 South-East SWAP (L9); PUSH (L3)
West L3 North-E SWAP( L7)
West L4 East PUSH (L1)
West L2 East POP
West L6 South-E PUSH (L2)
West L1 South-East SWAP (L9); PUSH (L1)

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.

Question 1: Packet received

Router R7 receives a packet with label L3:L4 from R2. What is the label stack of the outgoing packet that it sends ?

Question 2: Packet received

Router R7 receives a packet with label L7:L4 from R4. What is the label stack of the outgoing packet that it sends ?

Question 3: Packet received

Router R7 receives a packet with label L1 from R4. What is the label stack of the outgoing packet that it sends ?

Question 4: Packet received

Router R7 receives a packet with label L0:L5:L3 from R2. What is the label stack of the outgoing packet that it sends ?

Question 5: Packet received

Router R7 receives a packet with label L1:L5 from R4. What is the label stack of the outgoing packet that it sends ?