CCNP SP MPLS FAQ: Virtual Private LAN Services

CCNP SP MPLS FAQ: Virtual Private LAN Services

Q1. As which kind of port type can the customer-facing Ethernet interface on the PE router be configured?

Answer: The customer-facing Ethernet interface on the PE router can be configured as any of the following:

  • An access port
  • An 802.1Q trunk port
  • A dot1qtunnel port

Q2. How many labels are used to forward VPLS traffic, and what is the use of each of those labels?

Answer: Two labels are used to forward VPLS traffic. The top label, or tunnel label, identifies the tunnel (LSP) that the frame belongs to. In other words, it forwards the frame from the local or ingress PE to the remote or egress PE. The bottom label is the VC label. It identifies the pseudowire. In other words, the VC label is looked at by the remote PE; it uses the VC label to determine which attachment circuit the frame should be forwarded onto.

Q3. What does VFI stand for?

Answer: VFI stands for virtual forwarding instance.

Q4. Which Layer 2 control protocols can be tunneled across the VPLS network?

Answer: CDP, STP, and VTP can be tunneled across the VPLS network.

Q5. Why do the PE routers need to be in a full mesh of pseudowires in VPLS?

Answer: The PE routers need to be in a full mesh of pseudowires in VPLS because the chosen mechanism to prevent the looping of frames in the MPLS network is Layer 2 split-horizon, meaning that received frames on the pseudowires cannot be forwarded out on the pseudowires again.

Q6. Name the six functions that VPLS performs in emulating an Ethernet switch.

Answer: VPLS performs the following six functions in emulating an Ethernet switch:

  • Forwarding of Ethernet frames
  • Forwarding of unicast frames with an unknown destination MAC address
  • Replication of broadcast and multicast frames to more than one port
  • Loop prevention
  • Dynamic learning of MAC addresses
  • MAC address aging

Q7. In which two ways can H-VPLS be implemented?

Answer: H-VPLS can be implemented by using dot1q tunneling (QinQ) or MPLS pseudowires between the N-PEs and U-PEs in the access lay

Q8. Name the two major implementation models that describe Virtual Private Network (VPN) connectivity.

Answer: The Overlay model and the peer-to-peer, or network, model.

Q9. Name two possible business problems that can be solved by using a VPN.

Answer: Intra-company connectivity (intranet) and Inter-company connectivity (extranet).

Q10. In the peer-to-peer model, describe the difference between a C network and P network.

Answer: C network refers to the network infrastructure as managed by the VPN client. P network refers to the network infrastructure under control of the service provider.

Q11. Describe two advantages that the peer-to-peer model provides that the Overlay model does not.

ccnp-sp-mpls-faq-virtual-private-lan-services

Answer: Routing from a customer perspective becomes less complex because a routing adjacency must be maintained at only the entry the site-to-site traffic matrix. It’s simpler to add new sites because there is no need to provision virtual circuits between the sites.

Q12. Using the Overlay model, what is the formula to calculate the number of VCs required between a set of client sites if a full-mesh topology is deployed?

Answer: Number of VCs = [ (n – 1) * n) / 2 ] where n = number of attached devices.

Q13. List three technologies that provide IP-based Overlay VPN connectivity.

Answer: GRE, L2TP, and IPSec.

Q14. Why is the hub-and-spoke topology most often used when the Overlay model is deployed?

Answer: Primarily because of the cost of virtual circuits between sites. To reduce the cost of ownership, one or more hub sites are deployed with all spoke sites attaching through the hub sites.

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