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352-001 · Question #64

You are the lead network designer hired by Service Provider XYZ to deploy CoS functionality on the core MPLS network (P routers). The goal of the network design is to provide a complete CoS solution…

The correct answer is C. Map the appropriate DSCP value into the EXP field based on the number of queues in the MPLS. In an MPLS core network, DSCP values from customer traffic must be explicitly mapped to the MPLS EXP (Traffic Class) field so that P routers can apply the correct CoS treatment using only the label header.

Design Considerations

Question

You are the lead network designer hired by Service Provider XYZ to deploy CoS functionality on the core MPLS network (P routers). The goal of the network design is to provide a complete CoS solution to all customers that purchase services such as dedicated internet access, MPLS L3VPN, and L2VPN (pseudowire). Service Provider XYZ has these design requirements:

  • The network supports four service queues with equal treatment for delay, jitter, and packet loss.
  • Queues are numbered 0-3, where 0 is the default queue.
  • Three queues have one treatment.
  • One queue has either one or two treatments.

How would you design your solution to map the DSCP value properly so that the traffic is assigned to the respective queues in the Service Provider XYZ MPLS core network?

Options

  • AClassify traffic according to DSCP value into appropriate P router queues.
  • BMap the appropriate DSCP value into the EXP field based on the number of queues in the MPLS
  • CMap the appropriate DSCP value into the EXP field based on the number of queues in the MPLS
  • DBased on the DSCP value, traffic is mapped automatically into appropriate queues in the MPLS

How the community answered

(24 responses)
  • A
    29% (7)
  • B
    17% (4)
  • C
    46% (11)
  • D
    8% (2)

Why each option

In an MPLS core network, DSCP values from customer traffic must be explicitly mapped to the MPLS EXP (Traffic Class) field so that P routers can apply the correct CoS treatment using only the label header.

AClassify traffic according to DSCP value into appropriate P router queues.

P routers forward labeled packets and do not perform IP header lookups, so classifying traffic directly by DSCP value on P routers is not feasible without also mapping DSCP to the EXP field at ingress PE routers.

BMap the appropriate DSCP value into the EXP field based on the number of queues in the MPLS

While mapping DSCP to EXP is correct in principle, this option does not account for the queue with two treatments (e.g., two drop precedences within one queue), which requires a more granular EXP mapping than a simple one-to-one queue assignment.

CMap the appropriate DSCP value into the EXP field based on the number of queues in the MPLSCorrect

P routers in an MPLS network do not examine the IP header, so DSCP must be mapped to the 3-bit EXP/TC field at the PE ingress. With 4 queues (0-3) and one queue requiring two treatments (e.g., AF class with drop precedence), the mapping must account for up to 5 distinct EXP values, requiring careful DSCP-to-EXP configuration to preserve differentiated treatment across the core.

DBased on the DSCP value, traffic is mapped automatically into appropriate queues in the MPLS

MPLS does not automatically map DSCP values to EXP bits; this mapping must be explicitly configured on the ingress PE router using tools such as a DSCP-to-MPLS-EXP table or policy-map actions, with no default automatic behavior.

Concept tested: DSCP to MPLS EXP field mapping for CoS

Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/qos_classn/configuration/xe-16/qos-classn-xe-16-book/qos-classn-mrkg-mpls.html

Topics

#MPLS CoS#DSCP to EXP mapping#EXP field#service queues

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