300-410 · Question #5
What statement about route distinguishers in an MPLS network is true?
The correct answer is A. Route distinguishers make a unique VPNv4 address across the MPLS network. Route Distinguishers (RDs) are used in MPLS VPNs to make IPv4 prefixes from different VPNs unique when they are advertised within the MPLS backbone, forming a unique VPNv4 address.
Question
Options
- ARoute distinguishers make a unique VPNv4 address across the MPLS network.
- BRoute distinguishers allow multiple instances of a routing table to coexist within the edge router.
- CRoute distinguishers are used for label bindings.
- DRoute distinguishers define which prefixes are imported and exported on the edge router
How the community answered
(25 responses)- A92% (23)
- B4% (1)
- C4% (1)
Why each option
Route Distinguishers (RDs) are used in MPLS VPNs to make IPv4 prefixes from different VPNs unique when they are advertised within the MPLS backbone, forming a unique VPNv4 address.
In MPLS VPNs, Route Distinguishers (RDs) are prepended to IPv4 prefixes to create globally unique VPNv4 addresses. This allows multiple VPNs to use overlapping IPv4 address spaces without conflict when these prefixes are exchanged between Provider Edge (PE) routers across the MPLS core.
Route distinguishers do not allow multiple instances of a routing table to coexist; Virtual Routing and Forwarding (VRF) instances achieve this.
Route distinguishers are not directly used for label bindings; their role is in making the route unique for distribution, not in the label assignment process itself.
Route Targets (RTs), not Route Distinguishers (RDs), are used to define which VPNv4 prefixes are imported into and exported from a VRF on an edge router.
Concept tested: MPLS VPN Route Distinguisher function
Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mp_l3_vpns/configuration/xe-16/mp-l3-vpns-xe-16-book/mp-l3-vpns-vrfs.html
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