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352-001 · Question #211

You are tasked to design a QoS policy for a service provider so they can include it in the design of their MPLS core network. If the design must support an MPLS network with six classes, and CEs will

The correct answer is A. map DSCP bits into the Exp field. In an MPLS network where the SP manages CEs, DSCP markings from the CE should be mapped to the MPLS EXP (Traffic Class) field at the PE to carry QoS across the core.

Design Considerations

Question

You are tasked to design a QoS policy for a service provider so they can include it in the design of their MPLS core network. If the design must support an MPLS network with six classes, and CEs will be managed by the service provider, which QoS policy should be recommended?

Options

  • Amap DSCP bits into the Exp field
  • Bmap IP precedence bits into the DSCP field
  • Cmap flow-label bits into the Exp field
  • Dmap IP CoS bits into the IP Precedence field
  • Emap IP ToS bits into the Exp field

How the community answered

(42 responses)
  • A
    57% (24)
  • B
    5% (2)
  • C
    24% (10)
  • D
    12% (5)
  • E
    2% (1)

Why each option

In an MPLS network where the SP manages CEs, DSCP markings from the CE should be mapped to the MPLS EXP (Traffic Class) field at the PE to carry QoS across the core.

Amap DSCP bits into the Exp fieldCorrect

DSCP provides 6 bits of granularity with 64 possible values, which is more than sufficient to define six distinct traffic classes. Because the SP manages the CEs, DSCP markings can be enforced at ingress, and PE routers then map specific DSCP values to the 3-bit MPLS EXP (Traffic Class) field, enabling consistent end-to-end QoS treatment across the MPLS core.

Bmap IP precedence bits into the DSCP field

IP Precedence bits are a coarser subset of the DSCP byte; mapping IP Precedence into DSCP is the reverse of the correct direction and does not propagate QoS into the MPLS core.

Cmap flow-label bits into the Exp field

Flow-label is an IPv6 header field used for flow identification, not a QoS marking mechanism, and it is not mapped to the MPLS EXP field.

Dmap IP CoS bits into the IP Precedence field

IP CoS (802.1p) is a Layer 2 field and IP Precedence is Layer 3; mapping one to the other does not address MPLS core QoS and bypasses DSCP entirely.

Emap IP ToS bits into the Exp field

The IP ToS byte encompasses both DSCP and ECN bits; mapping the full ToS byte into the 3-bit EXP field is not a valid or standardized QoS mapping approach.

Concept tested: MPLS QoS DSCP to EXP field mapping for traffic classes

Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/qos_mqc/configuration/xe-16/qos-mqc-xe-16-book/qos-mpls-diffserv.html

Topics

#MPLS EXP#DSCP mapping#QoS#service classes

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