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Multiprotocol Label Switching

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Jun 25, 2008.

Chapter Description

This chapter introduces the core concepts with MPLS, particularly its use for unicast IP forwarding and for MPLS VPNs.

Blueprint topics covered in this chapter:

This chapter covers the following subtopics from the Cisco CCIE Routing and Switching written exam blueprint. Refer to the full blueprint in Table I-1 in the Introduction for more details on the topics covered in each chapter and their context within the blueprint.

  • Label Switching Router (LSR)
  • Label Switched Path (LSP)
  • Route Descriptor
  • Label Format
  • Label Imposition/Disposition
  • Label Distribution

Multiprotocol Label Switching (MPLS) remains a vitally important part of many service provider (SP) networks. MPLS is still growing in popularity in enterprise networks as well, particularly in larger enterprise internetworks. This chapter introduces the core concepts with MPLS, particularly its use for unicast IP forwarding and for MPLS VPNs.

"Do I Know This Already?" Quiz

Table 19-1 outlines the major headings in this chapter and the corresponding "Do I Know This Already?" quiz questions.

Table 19-1. "Do I Know This Already?" Foundation Topics Section-to-Question Mapping

Foundation Topics Section

Questions Covered in This Section


MPLS Unicast IP Forwarding




Other MPLS Applications


Total Score

In order to best use this pre-chapter assessment, remember to score yourself strictly. You can find the answers in Appendix A, "Answers to the 'Do I Know This Already?' Quizzes."

  1. Imagine a frame-based MPLS network configured for simple unicast IP forwarding, with four routers, R1, R2, R3, and R4. The routers connect in a mesh of links so that they are all directly connected to the other routers. R1 uses LDP to advertise prefix, label 30, to the other three routers. What must be true in order for R2 to advertise a label for to R1 using LDP?

    1. R2 must learn an IGP route to
    2. R2 will not advertise a label to R1 due to split horizon rules.
    3. R2 can advertise a label back to R1 before learning an IGP route to
    4. R2 must learn a route to using MP-BGP before advertising a label.
  2. In a frame-based MPLS network configured for unicast IP forwarding, LSR R1 receives a labeled packet, with a label value of 55. Which of the following could be true?

    1. R1 makes its forwarding decision by comparing the packet to the IPv4 prefixes found in the FIB.
    2. R1 makes its forwarding decision by comparing the packet to the IPv4 prefixes found in the LFIB.
    3. R1 makes its forwarding decision by comparing the packet to the MPLS labels found in the FIB.
    4. R1 makes its forwarding decision by comparing the packet to the MPLS labels found in the LFIB.
  3. R1, R2, and R3 are all MPLS LSRs that use LDP and connect to the same LAN. None of the three LSRs advertise a transport IP address. Which of the following could be true regarding LDP operation?

    1. The LSRs discover the other two routers using LDP Hellos sent to IP address
    2. Each pair of LSRs forms a TCP connection before advertising MPLS labels.
    3. The three LSRs must use their LAN interface IP addresses for any LDP TCP connections.
    4. The LDP Hellos use port 646, with the TCP connections using port 711.
  4. In a frame-based MPLS network configured for simple unicast IP forwarding, MPLS TTL propagation has been enabled for all traffic. Which of the following could be true?

    1. A traceroute command issued from outside the MPLS network will list IP addresses of the LSRs inside the MPLS network.
    2. A traceroute command issued from outside the MPLS network will not list IP addresses of the LSRs inside the MPLS network.
    3. Any IP packet with a TCP header, entering the MPLS network from outside the MPLS network, would not have its IP TTL field copied into the MPLS TTL field.
    4. An ICMP echo sent into the MPLS network from outside the MPLS network would have its IP TTL field copied into the MPLS TTL field.
  5. Which of the following is an extension to the BGP NLRI field?

    1. VRF
    2. Route Distinguisher
    3. Route Target
    4. BGP Extended Community
  6. Which of the following controls into which VRFs a PE adds routes when receiving an IBGP update from another PE?

    1. Route Distinguisher
    2. Route Target
    3. IGP metric
    4. AS Path length
  7. An ingress PE router in an internetwork configured for MPLS VPN receives an unlabeled packet. Which of the following is true?

    1. It injects a single MPLS header.
    2. It injects at least two MPLS headers.
    3. It injects (at least) a VPN label, which is used by any intermediate P routers.
    4. It uses both the FIB and LFIB to find all the required labels to inject before the IP header.
  8. An internetwork configured to support MPLS VPNs uses PHP. An ingress PE receives an unlabeled packet and then injects the appropriate label(s) to the packet before sending the packet into the MPLS network. Which of the following is/are true about this packet?

    1. The number of MPLS labels in the packet will only change when the packet reaches the egress PE router, which extracts the entire MPLS header.
    2. The number of MPLS labels in the packet will change before the packet reaches the egress PE.
    3. The PHP feature will cause the egress PE to act differently than it would without PHP enabled.
    4. None of the other answers is correct.
  9. Which of the following answers help define which packets are in the same MPLS FEC when using MPLS VPNs?

    1. IPv4 prefix
    2. ToS byte
    3. The MPLS VRF
    4. The TE tunnel
2. Foundation Topics | Next Section

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