350-501 Networking Practice Question
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?
⚠ Common exam trap
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.
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
✓
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.
Answer analysis
Option-by-option breakdown
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.
- ✗
Route target (RT)
Why it's wrong here
RT is used for VPN membership, not for making prefixes unique.
- ✗
IGP metric
Why it's wrong here
IGP metric is not added to MP-BGP VPN routes.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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