Understanding Next-Generation MVPN Concepts and Terminology

Route Distinguisher and VRF Route Target Extended Community

Route distinguisher and VPN routing and forwarding (VRF) route target extended communities are an integral part of unicast BGP-MPLS virtual private networks (VPNs). Route distinguisher and route target are often confused in terms of their purpose in BGP-MPLS networks. As they play an important role in BGP next-generation MVPNs, it is important to understand what they are and how they are used as described in RFC 4364.

RFC 4364 describes the purpose of route distinguisher as the following:

“A VPN-IPv4 address is a 12-byte quantity, beginning with an 8-byte Route Distinguisher (RD) and ending with a 4-byte IPv4 address. If several VPNs use the same IPv4 address prefix, the PEs translate these into unique VPN-IPv4 address prefixes. This ensures that if the same address is used in several different VPNs, it is possible for BGP to carry several completely different routes to that address, one for each VPN.”

Typically, each VRF table on a provider edge (PE) router is configured with a unique route distinguisher. Depending on the routing design, the route distinguisher can be unique or the same for a given VRF on other PE routers. A route distinguisher is an 8-byte number with two fields. The first field can be either an AS number (2 or 4 bytes) or an IP address (4 bytes). The second field is assigned by the user.

RFC 4364 describes the purpose of a VRF route target extended community as the following:

“Every VRF is associated with one or more Route Target (RT) attributes.

When a VPN-IPv4 route is created (from an IPv4 route that the PE router has learned from a CE) by a PE router, it is associated with one or more route target attributes. These are carried in BGP as attributes of the route.

Any route associated with Route Target T must be distributed to every PE router that has a VRF associated with Route Target T. When such a route is received by a PE router, it is eligible to be installed in those of the PE’s VRFs that are associated with Route Target T.”

The route target also contains two fields and is structured similar to a route distinguisher. The first field of the route target is either an AS number (2 or 4 bytes) or an IP address (4 bytes), and the second field is assigned by the user. Each PE router advertises its VPN-IPv4 routes with the route target (as one of the BGP path attributes) configured for the VRF table. The route target attached to the advertised route is referred to as the export route target. On the receiving PE router, the route target attached to the route is compared to the route target configured for the local VRF tables. The locally configured route target that is used in deciding whether a VPN-IPv4 route should be installed in a VRF table is referred to as the import route target.

C-Multicast Routing

Customer multicast (C-multicast) routing information exchange refers to the distribution of customer PIM (C-PIM) join/prune messages received from local customer edge (CE) routers to other PE routers (toward the VPN multicast source).

BGP MVPNs

BGP MVPNs use BGP as the control plane protocol between PE routers for MVPNs, including the exchange of C-multicast routing information. The support of BGP as a PE-PE protocol for exchanging C-multicast routes is mandated by Internet draft draft-ietf-l3vpn-mvpn-considerations-06.txt, Mandatory Features in a Layer 3 Multicast BGP/MPLS VPN Solution. The use of BGP for distributing C-multicast routing information is closely modeled after its highly successful counterpart of VPN unicast route distribution. Using BGP as the control plane protocol allows service providers to take advantage of this widely deployed, feature-rich protocol. It also enables service providers to leverage their knowledge and investment in managing BGP-MPLS VPN unicast service to offer VPN multicast services.

Sender and Receiver Site Sets

Internet draft draft-ietf-l3vpn-2547bis-mcast-10.txt describes an MVPN as a set of administrative policies that determine the PE routers that are in sender and receiver site sets.

A PE router can be a sender, a receiver, or both a sender and a receiver, depending on the configuration:

• A sender site set includes PE routers with local VPN multicast sources (VPN customer multicast sources either directly connected or connected via a CE router). A PE router that is in the sender site set is the sender PE router.

• A receiver site set includes PE routers that have local VPN multicast receivers. A PE router that is in the receiver site set is the receiver PE router.

Provider Tunnels

Internet draft draft-ietf-l3vpn-2547bis-mcast-10.txt defines provider tunnels as the transport mechanisms used for forwarding VPN multicast traffic across service provider networks. Different tunneling technologies, such as generic routing encapsulation (GRE) and MPLS, can be used to create provider tunnels. Provider tunnels can be signaled by a variety of signaling protocols. This topic describes only PIM-SM (ASM) signaled IP GRE provider tunnels and RSVP-Traffic Engineering (RSVP-TE) signaled MPLS provider tunnels.

In BGP MVPNs, the sender PE router distributes information about the provider tunnel in a BGP attribute called provider multicast service interface (PMSI). By default, all receiver PE routers join and become the leaves of the provider tunnel rooted at the sender PE router.

Provider tunnels can be inclusive or selective:

• An inclusive provider tunnel (I-PMSI provider tunnel) enables a PE router that is in the sender site set of an MVPN to transmit multicast data to all PE routers that are members of that MVPN.

• A selective provider tunnel (S-PMSI provider tunnel) enables a PE router that is in the sender site set of an MVPN to transmit multicast data to a subset of the PE routers.