This 350-501 practice question tests your understanding of networking. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A service provider is deploying MPLS L3VPN to connect multiple customer sites. The PE router receives a route from a CE router via MP-BGP. Which attribute must the PE router add to the route before advertising it to the route reflector?
MPLS label
Why wrong: The VPN label is assigned after the route is received.
Route distinguisher (RD)
RD makes the customer prefix unique across the MPLS VPN backbone.
Route target (RT)
Why wrong: RT is used for VPN membership, not for making prefixes unique.
IGP metric
Why wrong: IGP metric is not added to MP-BGP VPN routes.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Route distinguisher (RD)
When a PE router receives a customer route from a CE router via MP-BGP, it must first make the route unique within the MPLS VPN context by adding a Route Distinguisher (RD). The RD prepended to the IPv4 prefix creates a VPNv4 address (per RFC 4364), which is the format required for advertisement to a route reflector. Without the RD, the route would not be distinguishable from other customers' overlapping IP prefixes in the BGP table.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
MPLS label
Why it's wrong here
The VPN label is assigned after the route is received.
Route distinguisher (RD)
Why this is correct
RD makes the customer prefix unique across the MPLS VPN backbone.
Related concept
Read the scenario before looking for a memorised answer.
Route target (RT)
Why it's wrong here
RT is used for VPN membership, not for making prefixes unique.
IGP metric
Common exam traps
Cisco often tests the distinction between RD and RT, where candidates mistakenly think RT is required for route advertisement to the route reflector, but the RD is the attribute that makes the route globally unique in the VPNv4 address family.
Detailed technical explanation
The RD is an 8-byte value (e.g., 100:1) that, when combined with the IPv4 prefix, forms a 96-bit VPNv4 address, ensuring that overlapping customer prefixes remain unique in the BGP table. The route reflector receives only VPNv4 routes with RDs; it does not strip the RD but forwards the route to other PEs, which then use the RT to decide whether to import the route into a specific VRF. In real-world deployments, misconfiguration of RD assignment (e.g., using the same RD across different VRFs) can cause route collisions and blackholing.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the 350-501 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
The correct answer is: Route distinguisher (RD) — When a PE router receives a customer route from a CE router via MP-BGP, it must first make the route unique within the MPLS VPN context by adding a Route Distinguisher (RD). The RD prepended to the IPv4 prefix creates a VPNv4 address (per RFC 4364), which is the format required for advertisement to a route reflector. Without the RD, the route would not be distinguishable from other customers' overlapping IP prefixes in the BGP table.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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