350-501 Automation and Quality of Services Practice Question
In an SP MPLS core, a PE router maps incoming DSCP values to MPLS EXP bits. Which statement about EXP marking is accurate when using the DiffServ Tunneling Model?
⚠ Common exam trap
Cisco often tests the distinction between Pipe and Short Pipe models, where candidates mistakenly think the egress PE always uses EXP bits for queuing, but in Short Pipe the egress PE reverts to the original IP DSCP.
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
✓
In Short Pipe model, the egress PE uses the original IP DSCP for queuing decisions, not the EXP.
In the DiffServ Tunneling Model, the Short Pipe model specifically instructs the egress PE to ignore the MPLS EXP bits for queuing and instead use the original IP DSCP value. This preserves the end-to-end QoS marking across the MPLS core, making option D correct. The Pipe model, by contrast, uses the EXP bits at the egress PE, which is why option B is incorrect.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
EXP bits are always copied from the IP DSCP at the ingress PE.
Why it's wrong here
Only in Uniform model; in Pipe models, EXP is set separately.
- ✗
In Pipe model, the egress PE uses EXP bits for queuing.
Why it's wrong here
In Pipe, egress uses EXP; in Short Pipe, egress uses IP DSCP.
- ✗
The MPLS EXP field is 6 bits, same as DSCP.
Why it's wrong here
EXP is 3 bits (8 values).
- ✓
In Short Pipe model, the egress PE uses the original IP DSCP for queuing decisions, not the EXP.
Why this is correct
Short Pipe uses IP DSCP at egress, not EXP.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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