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CCNP SP MPLS FAQ: MPLS Architecture

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CCNP SP MPLS FAQ: MPLS Architecture

Q1. Name the four fields that are part of a label.

Answer: The four fields that are part of a label are a 20-bit label value, 3 experimental bits, 1 Bottom of Stack bit, and an 8-bit TTL field.

Q2. How many labels can reside in a label stack?

Answer: Any number of labels can reside in a label stack.

Q3. In which layer does MPLS fit in the OSI reference model?

Answer: MPLS fits in no category of the OSI reference model. The best description for MPLS would be Layer 2.5.

Q4. Which table does an LSR use to forward labeled packets?

Answer: The LSR uses the LFIB table to forward labeled packets.

Q5. What type of interfaces in Cisco IOS use the Downstream-on-Demand label distribution mode and the
per-interface label space?

Figure: Per-Interface Label Space

ccnp-sp-mpls-faq-mpls-architecture

Answer: LC-ATM interfaces use the Downstream-on-Demand label distribution mode and the per-interface label space.

Q6. Why does the MPLS label have a Time To Live (TTL) field?

Answer: As the packets are labeled, the IP TTL can no longer be used. A mechanism is still needed to avoid a packet circulating in a loop forever.

Q7. Describe some of the drawbacks that you might encounter when using traditional IP routing.

Answer:
Hop-by-hop forwarding decisions are based on IP destination lookup.IP lookup is based purely on the destination unicast address.

All packets bound for the same destination follow the same path.

All routers require full routing information to forward packets reliably.

Q8. Why is Policy Based Routing (PBR) unsuitable for engineering traffic in a large network?

Answer: PBR is a hop-by-hop mechanism and, therefore, cannot determine the entire path that a packet takes from ingress to egress. PBR is not scalable on a large scale because of its hop-by-hop nature; it also degrades router performance.

Q9. What benefit does the separation of forwarding and control elements, through the use of labels, provide?

Answer: A change in the information (such as a label) that controls the forwarding of packets does not need to be communicated to all devices within the network.

Q10. Does every MPLS node need to run an IP routing protocol to exchange IP routing information with other MPLS nodes?

Answer: Yes. IP reachability of other MPLS nodes must be determined and, therefore, requires an IP routing protocol, or static routing.

Q11. List the different types of LSRs within an MPLS network.

Answer: LSR, Edge-LSR, ATM LSR, and ATM Edge-LSR.

Q12. Describe the functionality of an Edge-LSR.

Answer: An Edge-LSR is a router that performs label imposition/disposition at the edge of an MPLS domain and has one or more non-MPLS neighbors.

Q13. What is a Label Switched Path (LSP) and is it unidirectional or bidirectional?

Answer: An LSP is the combination of labels that can switch a packet from the ingress to egress of an MPLS network. An LSP is always unidirectional.

Q14. Is Cisco Express Forwarding (CEF) necessary on an Edge-LSR?

Answer: Yes. CEF is the only switching mechanism on a Cisco router that builds an FIB, which is required for label imposition.

Q15. Which two tables does the LSR use to hold information that is relevant to the MPLS forwarding component?

Answer: Label Information Base (LIB). Label Forwarding Information Base (LFIB).

Q16. What is the primary difference between the Label Forwarding Information Base (LFIB) and the Label Information Base (LIB)?

Answer: The LFIB holds only the labels currently in use by the MPLS forwarding component, whereas the LIB holds all labels received from all neighbors.

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