According to Juniper documentation, a route distinguisher (RD) is a critical address qualifier used to ensure that network layer reachability information (NLRI) remains unique within a Multiprotocol BGP (MP-BGP) control plane, especially in multitenant environments. In a service provider network, different tenants may use identical, overlapping private IP address spaces (such as 192.168.1.0/24). If these routes were advertised without a qualifier, BGP—which typically only considers the IP prefix—would treat them as the same route and potentially overwrite one with the other based on standard BGP best-path selection.

The route distinguisher solves this by prepending an 8-byte field to the tenant's IP address, effectively transforming a 32-bit IPv4 prefix into a unique 96-bit VPN-IPv4 address. This 8-byte value is typically configured in formats such as as-number:number or ip-address:number. Because the resulting VPN-IPv4 prefix is unique to that specific routing instance, the service provider's BGP infrastructure can carry and distinguish between overlapping routes from multiple customers simultaneously.

It is important to differentiate the RD from a route target (RT). While the RD's sole purpose is to provide uniqueness so that routes are not discarded or overwritten, the route target is an extended community attribute used to control the import and export of these routes into the correct virtual routing and forwarding (VRF) tables on remote PE routers. In Juniper's Junos OS, each routing instance of type vrf or evpn must have a unique RD associated with it; failing to configure a unique RD or attempting to use the same RD across different instances will result in a configuration commit failure. This fundamental separation of "making routes unique" (RD) and "directing routes to the right place" (RT) allows for the scalable, isolated multitenancy required in modern EVPN-VXLAN campus and data center fabrics.