The Route Distinguisher (RD)

The route distinguisher is a 64 bit identifier attribute that gets prepended to the user IPv4 or IPv6 address and makes the IP address globally unique across the VPN network and within the BGP routing table. The RD is a required component when defining a L3 VPN. Its significance is local to the device. Assign one RD to each VRF that will use the L3 VPN. The BGP VPN-IPv4 or VPN-IPv6 address families are defined by combining the RD, user IP address, and the MPLS Label (see MPLS Label Mode for label mode configuration).

RDs must be unique for each L3 VPN VRF on a device. The same RD can be used on multiple devices belonging to the VPN. Combining the VRF RD with the user IP address, even when that IP address is an unregistered private address, serves to uniquely identify the user.

Three data fields make up the eight bytes (64-bits) of the RD attribute:

• RD Type – A non-configurable two-byte field that identifies the format used by the administrator and assigned fields as the packet transits the network. Valid values are 0, 1, or 2.
• Administrator Field – A two- or four-byte field (depending upon the RD type) allowing a network administrator to uniquely identify the VRF as a:
  • Two-byte autonomous system number (RD type 0). Valid values are 1 - 65535.
  • Four-byte IPv4 address (RD type 1)
  • Four-byte autonomous system number (RD type 2). Valid values are 65536 - 4294967295.
• Assigned Number Field – A two- or four-byte field (depending upon the RD type) assigned by the provider network:
  • Four-byte autonomous system number (RD type 0). Valid values are 1 - 4294967295.
  • Two-byte autonomous system number (RD types 1 and 2). Valid values are 1 - 65535.

It is recommended that non-private autonomous system numbers be used when configuring the RD. If the BGP autonomous system number is a private AS between 64512-65534, use RD type 1 specifying an IPv4 address.

Non-private autonomous system numbers are assigned by IANA to service providers. Non-private autonomous system numbers use either a two-byte or four-byte number in the following formats:

• Type 0 – 1 - 65535:1 - 4294967295
• Type 1 – IPv4-address:1 - 65535
• Type 2 – 65536 - 4294967295:1 - 65535

This example shows how to assign a type 0 route distinguisher 1:52 to VRF vpnA:

System(rw)->router vpnA
System(su-vpnA)->configure
System(su-vpnA-config)->rd 1:52
System(su-vpnA-config)->

This example shows how to assign a type 1 route distinguisher 10.10.100.1:53 to VRF vpnB:

System(rw)->router vpnB
System(su-vpnB)->configure
System(su-vpnB-config)->rd 10.10.100.1:53
System(su-vpnB-config)->