Route Distinguisher (RD) Explained & Configuration

Route Distinguisher concept

Assigning a route distinguisher (RD) for the VRF is required to identify a virtual routing domain in a provider’s network and allows for overlapping IP space. The route distinguisher must be a unique value on the router for each VRF.

The IP prefix is a member of the IPv4 address family. After the PE device learns the IP prefix, the PE converts it into a VPN-IPv4 prefix by combining it with an 8-byte route distinguisher (RD).

The resulting 12-octet field is a unique “VPN-IPv4” address. The route distinguisher used to generate the VPN-IPv4 prefix is specified by a configuration command associated with the virtual routing and forwarding (VRF) instance on the PE device.

VPNv4 subnet format:

VPN customer IPv4 prefix is converted into a VPNv4 prefix by appending the RD (1:1,
say) to the IPv4 address (200.1.64.0, say) => 1:1:200.1.64.0
• Makes the customer’s IPv4 address unique inside the SP MPLS network.
• Route Distinguisher (rd) is configured in the VRF at PE
• RD is not a BGP attribute, just a field.

The route distinguisher has only one purpose: to make IPv4 prefixes globally unique. It is not used for IP forwarding by the provider’s core (non-edge) routers (within the MPLS cloud), but it is used by the PE to identify which VPN a packet belongs to.

In other words, it allows the router install routes from differents customer (could be overlapping subnets) in the BGP table (the subnet is identified uniquely for each customer/VRF thanks to added 8 bytes RD).

For example, for a PE router to be able to distinguish between the IP address 10.2.0.0 of one customer from the 10.2.0.0 of another customer, the network administrator must configure the PE to add a unique route distinguisher to each packet arriving from the CEs.

Basic configuration and Verification:

A route distinguisher must be configured for the virtual routing and forwarding (VRF) instance. The purpose of using an RD in a VRF declaration is to put unique IPv4 prefixes into the BGP common global table in the PE.

switch-1(config)# ip vrf Customer-A
switch-1(config-vrf)# rd 65001:101Code language: PHP (php)

Use the show ip vrf command to verify the route distinguisher (RD) and interface that are configured for the VRF.