CCNP SP MPLS FAQ: MPLS and Quality of Service
Q1. How many bits in the IP header can be used for QoS?
Answer: Three bits in the IP header can be used for QoS if the precedence bits are used, and six bits can be used if the DSCP bits are used.
Q2. How many AF classes exist?
Answer: Four AF classes exist.
Q3. Name the three MPLS DiffServ models.
Answer: Uniform model, Pipe model, and Short Pipe model are the three MPLS DiffServ models.
Q4. What is the difference between the Pipe and Short Pipe models?
Answer: The only difference between the two models is the forwarding treatment of the packets on the egress LSR. The egress LSR forwards packets based on the LSP DiffServ information in the Pipe model. The egress LSR forwards packets based on the Tunneled DiffServ information in the Short Pipe model.
Q5. What is the interface command to encapsulate all IP packets with an explicit NULL label?
Answer: mpls ip encapsulate explicit-null is the interface command to encapsulate all IP packets with an explicit NULL label.
Q6. What is TOS reflection?
Answer: TOS reflection is the default behavior of copying the precedence value of an IP packet to the EXP bits of all imposed labels.
Q7. What feature is used in Cisco IOS to alter the EXP bits?
Answer: MQC is used in Cisco IOS to alter the EXP bits.
Q8. What is the problem with PHP and QoS?
Answer: The problem with PHP and QoS is that the EXP bits value is not copied to the newly exposed MPLS label or precedence/DiffServ bits in the IP header, if the packet becomes unlabeled.
Q9. What is the solution to the problem in question 8?
Answer: The solution is to copy the EXP bits value by using qos-group or the explicit NULL label.
Q10. On which labels can you change the EXP bits value?
Answer: You can change the EXP bits value only on the top label and the pushed label(s).