CCNP SP MPLS FAQ: The Evolution of MPLS

CCNP SP MPLS FAQ: The Evolution of MPLS

Q1. What are the MPLS applications mentioned in this chapter?

Answer: Traffic engineering, MPLS VPN, AToM, and VPLS are the MPLS applications mentioned in this chapter.

Q2. Name three advantages of running MPLS in a service provider network.

Answer: The advantages of running MPLS in a service provider network include the following:

  • Better integration of IP over ATM
  • A network that is running a BGP-free core
  • Easy deployment of a peer-to-peer VPN model (MPLS VPN)
  • One unified network infrastructure
  • Optimal flow of traffic
  • Traffic engineering enables the steering of traffic through the network on a path different than the least cost path computed by the dynamic routing protocol

Q3. What are the advantages of the MPLS VPN solution for the service provider over all the other VPN solutions?

Answer: MPLS VPN allows for easy provisioning of sites and allows for optimal traffic flow in the backbone network at all times.

Q4. Name the four technologies that can be used to carry IP over ATM.

Answer: The four technologies that can be used to carry IP over ATM are as follows:
  • RFC 1483
  • LANE
  • MPOA
  • MPLS

Q5. Name two pre-MPLS protocols that use label switching.

Answer: ATM and Frame Relay are two pre-MPLS protocols that use label switching.

Q6. What do the ATM switches need to run so that they can operate MPLS?

Answer: The ATM switches need an IP routing protocol and a label distribution protocol to operate MPLS.

Q7. How do you ensure optimal traffic flow between all the customer sites in an ATM or Frame Relay overlay network?

Figure : Non-Fully Meshed Overlay ATM Network

ccnp-sp-mpls-faq-evolution-mpls

Answer: The connectivity between the customer sites needs to be a full mesh of virtual circuits.

Q8. MPLS can function using two different modes. What are they?

Answer: Frame-mode—Labeled packets are exchanged as frames at Layer 2

Q9. When using frame-mode MPLS, where is the label header carried within the frame?

Answer: The label header is inserted between the Layer-2 and Layer-3 headers.

Q10. Describe the format of an MPLS label header.

Answer:
20 bits label
3 bits EXP field
1 bit S
8 bits TTL field

Q11. In a LAN environment, how does the receiving router identify an MPLS packet?

Answer: Labeled packets can be identified by their ethertype value of 0x8847 or 0x8848.

Q12.: List three different actions an MPLS LSR can perform on a labeled packet when running in frame-mode.

Answer:
Pop—Removes the top label and forwards as either an IP or MPLS labeled packet.
Swap—Replaces the top label with another value.
Push—Places another label or labels on the top of the label stack.
Aggregate—The egress LSR has to perform additional L3 lookup in its IP forwarding table to successfully forward the packet.
Untag—Similar to Pop, but the result of the Pop operation is an unlabeled IP datagram. This operation is used on egress circuits toward non-MPLS nodes.

Q13. Name two label-distribution protocols used in conjunction with unicast destination-based routing.

Answer:
Tag Distribution Protocol (TDP).
Label Distribution Protocol (LDP).

Q14. Which label distribution method does a frame-mode implementation use?

Answer: Unsolicited downstream label distribution.

Q15. Which label allocation method does a frame-mode implementation use?

Answer: Independent control label allocation.

Q16. Which label retention method does a frame-mode implementation use?

Answer: Liberal label retention.

Q17. How is penultimate hop popping requested through the use of TDP or LDP?

Answer: By using an implicit-null label value to indicate that the upstream LSR should remove the top label from the MPLS label stack.

Q18. Because IP packets cannot be exchanged directly between two adjacent

Answer: A control-VC is necessary so that control plane information can be exchanged.

Q19. What is an LC-ATM interface?

Answer: Any interface on a router or ATM switch in which the VPI/VCI values are assigned through an MPLS control protocol.

Q20. Which label-distribution method is utilized across an LC-ATM interface?

Answer: Downstream-on-demand label distribution.

Q21. Which label-allocation method does a cell-mode implementation use?

Answer: Ordered control label allocation.

Q22. Which label retention method does a cell-mode implementation use?

Answer: Conservative label retention.

Q23. What is the benefit of using VC-merge?

Answer: Using VC-merge reduces the amount of label space an ATM-LSR requires.

Q24. Why are labels not distributed to upstream neighbors without having received a label mapping from a downstream neighbor?

Answer: To make sure that a complete LFIB entry is available to facilitate correct label switching through the ATM switching matrix.

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Prasanna

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