Understanding the Role of the Route Distinguisher (RD) in Layer 3 VPNs

B

In a Layer 3 VPN, the role of the route distinguisher (RD) is to enable the creation of unique VPNv4 prefixes. VPNv4 prefixes are IPv4 addresses that are used to represent routes within a VPN. These prefixes are unique to each VPN, and are used to ensure that traffic is routed correctly between different VPNs.

The RD is a 64-bit value that is attached to the IPv4 prefix to create a unique VPNv4 prefix. The RD is added to the front of the IPv4 prefix to create a 96-bit VPNv4 prefix, which is then used to identify the route within the VPN. By adding the RD to the IPv4 prefix, it becomes possible to have different VPNs with the same IPv4 address space.

Option B is the correct answer. The RD allows different VRFs to contain the same IP prefixes. This is because the RD is used to create unique VPNv4 prefixes for each VRF, even if they contain the same IPv4 address space. This enables multiple VPNs to coexist on the same physical infrastructure, without interfering with each other.

Option A is incorrect because MPLS labels are allocated by the Label Distribution Protocol (LDP) or RSVP-TE, not the RD. Option C is incorrect because the RD is used to create unique VPNv4 prefixes within the VRF, not within the global routing instance. Option D is incorrect because the import and export of routes within a VRF is controlled by the route target (RT), not the RD.