CCNP SP MPLS FAQ: Forwarding Labeled Packets

CCNP SP MPLS FAQ: Forwarding Labeled Packets

Q1. What does the push operation do on a labeled packet?

Answer: The push operation replaces the top label with another and then pushes one or more labels onto the label stack.

Q2. Which Cisco IOS command do you use to see what the swapped label is and which labels are pushed onto a received packet for a certain prefix?

Answer: show mpls forwarding-table [network { mask | length}] [detail]

Q3. What does the outgoing label entry of “Aggregate” in the LFIB of a Cisco IOS LSR mean?

Answer: The outgoing label entry “Aggregate” means that the LSR removes the label and does an IP lookup to be able to determine where the packet needs to be forwarded.

Q4. What label value signals the penultimate LSR to use penultimate hop popping (PHP)?

Figure: Penultimate Hop Popping


Answer: The label with value 3, known as the implicit NULL label, signals the penultimate LSR to use PHP.

Q5. What are the value and the function of the Router Alert label?

Answer: The value of the Router Alert label is 1 and its function is to make sure that all LSRs forwarding this packet take a closer look at it.

Q6. Why does an LSR forward the ICMP message “time exceeded” along the LSP of the original packet with the TTL expiring instead of returning it directly?

Answer: The LSR—or an intermediate LSR—might not have the information it needs to return the ICMP message, so it forwards it along the LSP in the hope that the packet reaches a router that can return the ICMP message to the sender of the original packet.

Q7. Is using Path MTU Discovery a guarantee that there will be no MTU problems in the MPLS network?

Answer: No, the ICMP messages might not make it back to the originator of the packet for various reasons.

Q8. Why is MTU or MRU such an important parameter in MPLS networks?

Answer: The MRU is so important in MPLS networks because the addition of a label stack increases the size of a frame slightly. As such, the size of the frame might become more than the maximum allowed size of a frame on the data link. Especially on Ethernet links, care must be taken that such frames can still be forwarded. Such frames are known as baby giant frames.

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