CCNP SP MPLS FAQ: MPLS and ATM Architecture

CCNP SP MPLS FAQ:MPLS and ATM Architecture

Q1. In what ways is an ATM LSR different from a frame-based LSR?

  • The label value is encoded in the VPI/VCI fields.
  • ATM LSRs forward cells.
  • ATM LSRs are not capable of decrementing the TTL.
  • LDP runs in Downstream-on-Demand label advertisement mode on ATM LSRs.

Q2. What is the default control VC for LDP?

Answer: 0/32 is the default control VC for LDP.

Q3. What is the preferred control mode for LDP on the ATM LSRs?

Answer: Ordered Control mode is the preferred mode for LDP on the ATM LSRs.

Q4. Name two ways that LDP can detect loops.

Answer: LDP can detect loops in the following two ways:
By means of a Hop Count TLV
By means of a Path Vector TLV

Q5. Which two features must an ATM switch have in the control plane to become an ATM LSR?

Answer: An ATM LSR must have an IGP and a label distribution protocol.

Q6. Which label space is used on an LC-ATM interface?

Answer: Per-interface label space is used on an LC-ATM interface.

Q7. What IP precedence values are by default mapped to the standard LVC type with Multi-VC TBR?

Answer: Precedence values 1 and 5 are mapped by default to the standard` LVC type with Multi-VC TBR.

Q8. What would be the reason to disable the head end VCs on an LSC?

Answer: You might disable the head end VCs on an LSC to reduce the number of LVCs through the ATM network.

Q9. Why does the Cisco equipment not advertise bound IP addresses of the LSR on LC-ATM interfaces with LDP?

Answer: The Cisco equipment does not advertise these addresses because they are not needed when the LSRs are running LDP on LC-ATM links. Mapping the received label to the downstream LDP peer is clear.

Q10. What is the advantage and disadvantage of VC-Merge?

Figure: VC-Merge


Answer: The advantage of VC-Merge is the reduction of LVCs; the disadvantage is the buffering of the cells, which implies the need for more memory on the LSR.

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