350-501 Automation and Quality of Service Practice Question
In an MPLS network, which field in the MPLS label header is used to carry QoS information between LSRs?
⚠ Common exam trap
Cisco often tests the distinction between IP-layer QoS fields (ToS, IP Precedence, DSCP) and MPLS-layer QoS fields (EXP bits), so the trap here is that candidates mistakenly choose DSCP or IP Precedence because they are familiar QoS markings, forgetting that MPLS uses its own label header field for QoS between LSRs.
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
✓
EXP bits
In MPLS, the EXP (Experimental) bits, also known as the Traffic Class (TC) field per RFC 5462, are 3 bits in the MPLS label header used to carry Quality of Service (QoS) information between Label Switch Routers (LSRs). These bits allow LSRs to apply per-hop behaviors (PHBs) such as queuing and scheduling based on the packet's QoS class, enabling differentiated services across the MPLS network.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
ToS
Why it's wrong here
ToS is the same as DSCP/IP Precedence in the IP header.
- ✗
IP Precedence
Why it's wrong here
IP Precedence is in the IP header.
- ✓
EXP bits
Why this is correct
EXP bits are used for QoS in MPLS.
- ✗
DSCP
Why it's wrong here
DSCP is in the IP header, not the MPLS label.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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