200-901 Network Fundamentals Practice Question
A network operations team is deploying a new application that requires guaranteed bandwidth and low latency between two data centers. The team decides to use MPLS Layer 3 VPN. Which component of MPLS is responsible for forwarding packets based on labels rather than traditional IP routing lookups?
⚠ Common exam trap
Watch out — candidates often confuse the roles of edge and core MPLS devices; the LER assigns labels, but the LSR is the one that forwards based on labels.
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
✓
Label Switch Router (LSR)
In MPLS, the Label Switch Router (LSR) is the device that forwards packets based on labels. It uses the Label Forwarding Information Base (LFIB) to determine the outgoing interface and new label. This allows MPLS networks to make forwarding decisions without examining the IP header, enabling traffic engineering and VPNs. The LERs handle label imposition and disposition at the edges.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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Label Edge Router (LER)
Why it's wrong here
The Label Edge Router (LER) operates at the edge of an MPLS network and is responsible for pushing and popping labels. It classifies packets and assigns them to a Forwarding Equivalence Class (FEC). While it initiates label switching, the actual forwarding based on labels within the core is performed by Label Switch Routers (LSRs).
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Autonomous System Boundary Router (ASBR)
Why it's wrong here
An ASBR is a router that connects an OSPF area to other routing domains or autonomous systems. It is not specific to MPLS label forwarding; it may participate in MPLS but its primary role is routing between ASes. It does not perform the core label-swapping function described.
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Route Reflector
Why it's wrong here
A Route Reflector is a BGP concept used to reduce the number of IBGP sessions required in a network. It does not forward MPLS packets based on labels; it only helps distribute routing information. It operates at the control plane, not the data plane, and is not involved in label switching.
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Label Switch Router (LSR)
Why this is correct
The Label Switch Router (LSR) is a core MPLS device that forwards packets based on labels. It examines the incoming label, looks up the next-hop in its Label Forwarding Information Base (LFIB), swaps the label, and forwards the packet. This label-based forwarding is faster and more efficient than traditional IP routing lookups.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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