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352-001 · Question #214

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 t

The correct answer is B. Traffic is classified on the MPLS PE routers on core facing interface. The DSCP value is mapped. In MPLS networks, QoS classification occurs at the PE router's core-facing interface where DSCP values are mapped to MPLS EXP (Traffic Class) bits before traffic enters the P routers.

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.

If your design includes eight CoS queues on the Service Provider XYZ MPLS PE router ingress (CE facing) interface, how will customer traffic be classified as it enters the MLS P routers?

Options

  • AThe eight CoS queues in the MPLS P router are remapped to the eight CoS queues.
  • BTraffic is classified on the MPLS PE routers on core facing interface. The DSCP value is mapped
  • CDiscard the traffic from the eight CoS queues that does not match the four CoS queues of the
  • DThe 8 CoS queues in the MPLS P router are remapped to four 4 flow-label queues.

How the community answered

(25 responses)
  • A
    28% (7)
  • B
    56% (14)
  • C
    12% (3)
  • D
    4% (1)

Why each option

In MPLS networks, QoS classification occurs at the PE router's core-facing interface where DSCP values are mapped to MPLS EXP (Traffic Class) bits before traffic enters the P routers.

AThe eight CoS queues in the MPLS P router are remapped to the eight CoS queues.

P routers do not maintain eight CoS queues - they operate on MPLS EXP bits which are only 3 bits wide, and the design specifies four queues on the core, not eight.

BTraffic is classified on the MPLS PE routers on core facing interface. The DSCP value is mappedCorrect

When traffic arrives at the PE ingress from the CE side, the PE router classifies it and maps DSCP values to MPLS EXP bits on its core-facing interface. P routers in the MPLS core use these 3-bit EXP fields for QoS treatment, honoring the four-queue design without needing to inspect the original IP DSCP values. This allows the eight PE ingress queues to be reduced to the four core queues through DSCP-to-EXP mapping at the PE.

CDiscard the traffic from the eight CoS queues that does not match the four CoS queues of the

Traffic is not discarded - it is reclassified via DSCP-to-EXP mapping at the PE core-facing interface to fit the four-queue model before entering the P routers.

DThe 8 CoS queues in the MPLS P router are remapped to four 4 flow-label queues.

Flow labels are an IPv6 header concept and are not a QoS queuing mechanism in MPLS; MPLS cores use EXP/TC bits, not flow-label queues, for traffic classification.

Concept tested: MPLS QoS EXP bit mapping at PE routers

Source: https://www.cisco.com/c/en/us/support/docs/multiprotocol-label-switching-mpls/mpls/13401-mpls-qos-dsc.html

Topics

#MPLS CoS#queue remapping#PE classification#service queues

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