Route Distinguishers
To support customers with overlapping IP address space, MPLS providers
use route distinguishers (RD). An RD is a 64-bit prefix added to each
customer’s IP address to make it globally unique. The resulting 96-bit IP
address is called a VPNv4 address.
RDs are used to propagate routes across provider networks in the following
way:
Step 1. A C router advertises its normal IPv4 networks to the PE router.
Step 2. The PE router prepends the RD to the C networks to create
VPNv4 addresses.
Step 3. The PE router advertises the VPNv4 addresses to its MP-BGP
peers.
Step 4. Other PE routers strip the RD from the network address and
advertise the route to C routers in the same VRF.
Note
The RD is used only within the MPLS network and only to create a globally unique address. C
routers never see the RD and are not aware of their VPNv4 address.