350-501 Automation and Quality of Services Practice Question
In the context of MPLS QoS, at which point are EXP bits typically set based on the DSCP value of the IP packet?
⚠ Common exam trap
Cisco often tests the misconception that EXP marking happens at the egress PE or that P routers can modify EXP bits based on routing, but the key is that EXP bits are set only at the ingress PE when the label is imposed, and P routers treat the EXP field as read-only unless explicit EXP manipulation is configured (e.g., via 'mpls experimental' in a policy-map on the P router, which is rare).
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
At the ingress PE router before the packet enters the MPLS core.
In MPLS QoS, the EXP bits are set at the ingress PE router because this is where the IP packet first encounters the MPLS domain. The ingress PE maps the incoming IP packet's DSCP value to the MPLS EXP bits using a configured trust boundary or marking policy (e.g., 'policy-map' with 'set mpls experimental imposition'). This ensures that the QoS treatment is applied consistently across the MPLS core, as P routers typically forward based on the EXP bits without inspecting the original IP header.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
At the P routers in the core based on routing protocols.
Why it's wrong here
P routers typically trust the EXP bits already set; they do not map DSCP to EXP.
- ✓
At the ingress PE router before the packet enters the MPLS core.
Why this is correct
The ingress PE maps DSCP to EXP bits.
- ✗
At the egress PE router when packets are forwarded to the CE.
Why it's wrong here
EXP bits are set at the ingress PE, not egress.
- ✗
At the CE router before sending to the PE.
Why it's wrong here
CE may set DSCP but not MPLS EXP bits; EXP bits are set at the PE.
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Senior Network & Security Engineer · founder of Courseiva
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