How to Ensure Unique Customer Prefixes in Layer 3 VPNs
Question
You have multiple Layer 3 VPNs supporting your various customers. Some of the customers use overlapping IP address space.
What is used to ensure each customers prefixes are unique?
Answers
Explanations
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A. B. C. D.D
A route distinguisher (RD) distinguishes one set of routes (one VRF) from another. It is a unique number prepended to each route within a VRF to identify it as belonging to that particular VRF or customer. An RD is carried along with a route via MP-BGP when exchanging VPN routes with other PE routers.
Incorrect Answers:
A: Route targets can be used to share routes.
http://packetlife.net/blog/2013/jun/10/route-distinguishers-and-route-targets/When multiple Layer 3 VPNs support different customers, it is common for some of them to use overlapping IP address space. To ensure that each customer's prefixes are unique, a mechanism called Route Distinguishers (RDs) is used.
RDs are used to distinguish one customer's set of prefixes from another customer's prefixes. It is a unique identifier that is added to each customer's VPN-IPv4 prefix to make it unique across the service provider's MPLS network.
The RD is added to the front of the customer's IPv4 address to form a globally unique VPN-IPv4 address. This VPN-IPv4 address is then advertised to the MPLS network through the use of Multiprotocol BGP (MP-BGP).
MP-BGP is used to exchange VPN-IPv4 prefixes between the customer's CE routers and the service provider's PE routers. When the PE router receives a VPN-IPv4 prefix from a CE router, it associates the prefix with a particular VPN based on the RD.
In addition to RDs, other mechanisms such as Route Targets (RTs) and BGP communities are also used to control the distribution of VPN routes within the MPLS network. However, these mechanisms are not used to ensure the uniqueness of VPN-IPv4 prefixes.
Therefore, the correct answer to the question is D, Route Distinguisher (RD).
