2. The VPN-IPv6 Address Family

The BGP Multiprotocol Extensions [BGP-MP] allow BGP to carry routes from multiple "address families". We introduce the notion of the "VPN-IPv6 address family", which is similar to the VPN-IPv4 address family introduced in [BGP/MPLS-VPN].

A VPN-IPv6 address is a 24-octet quantity, beginning with an 8-octet "Route Distinguisher" (RD) and ending with a 16-octet IPv6 address.

The purpose of the RD is solely to allow one to create distinct routes to a common IPv6 address prefix, which is similar to the purpose of the RD defined in [BGP/MPLS-VPN]. In the same way as it is possible per [BGP/MPLS-VPN], the RD can be used to create multiple different routes to the very same system. This can be achieved by creating two different VPN-IPv6 routes that have the same IPv6 part but different RDs. This allows the provider's BGP to install multiple different routes to the same system and allows policy to be used to decide which packets use which route.

Also, if two VPNs were to use the same IPv6 address prefix (effectively denoting different physical systems), the PEs would translate these into unique VPN-IPv6 address prefixes using different RDs. This ensures that if the same address is ever used in two different VPNs, it is possible to install two completely different routes to that address, one for each VPN.

Since VPN-IPv6 addresses and IPv6 addresses belong to different address families, BGP never treats them as comparable addresses.

A VRF may have multiple equal-cost VPN-IPv6 routes for a single IPv6 address prefix. When a packet's destination address is matched in a VRF against a VPN-IPv6 route, only the IPv6 part is actually matched.

The Route Distinguisher format and encoding is as specified in [BGP/MPLS-VPN].

When a site is IPv4 capable and IPv6 capable, the same RD MAY be used for the advertisement of IPv6 addresses and IPv4 addresses. Alternatively, a different RD MAY be used for the advertisement of the IPv4 addresses and of the IPv6 addresses. Note, however, that in the scope of this specification, IPv4 addresses and IPv6 addresses will always be handled in separate contexts, and that no IPv4-IPv6 interworking issues and techniques will be discussed.