Next-Generation MVPN Data Plane Overview

A next-generation multicast virtual private network (MVPN) data plane is composed of provider tunnels originated by and rooted at the sender provider edge (PE) routers and the receiver PE routers as the leaves of the provider tunnel.

A provider tunnel can carry data for one or more VPNs. Those provider tunnels that carry data for more than one VPN are called aggregate provider tunnels and are outside the scope of this topic. Here, we assume that a provider tunnel carries data for only one VPN.

This topic covers two types of tunnel technologies: IP generic routing encapsulation (GRE) provider tunnels signaled by Protocol Independent Multicast-Sparse Mode (PIM-SM) any-source multicast (ASM) and MPLS provider tunnels signaled by RSVP-Traffic Engineering (RSVP-TE).

When a provider tunnel is signaled by PIM, the sender PE router runs another instance of the PIM protocol on the provider’s network (P-PIM) that signals a provider tunnel for that VPN. When a provider tunnel is signaled by RSVP-TE, the sender PE router initiates a point-to-multipoint label-switched path (LSP) toward receiver PE routers by using point-to-multipoint RSVP-TE protocol messages. In either case, the sender PE router advertises the tunnel signaling protocol and the tunnel ID to other PE routers via BGP by attaching the provider multicast service interface (PMSI) attribute to either the Type 1 intra-AS autodiscovery routes (inclusive provider tunnels) or Type 3 S-PMSI autodiscovery routes (selective provider tunnels).

The PMSI attribute contains the provider tunnel type and an identifier. The value of the provider tunnel identifier depends on the tunnel type. Table 1 identifies the tunnel types specified in Internet draft draft-ietf-l3vpn-2547bis-mcast-bgp-08.txt.

Selective Provider Tunnels (S-PMSI Autodiscovery/Type 3 and Leaf Autodiscovery/Type 4 Routes)

A selective provider tunnel is used for mapping a specific C-multicast flow (a (C-S, C-G) pair) onto a specific provider tunnel. There are a variety of situations in which selective provider tunnels can be useful. For example, they can be used for putting high-bandwidth VPN multicast data traffic onto a separate provider tunnel rather than the default inclusive provider tunnel, thus restricting the distribution of traffic to only those PE routers with active receivers.

In BGP next-generation multicast virtual private networks (MVPNs), selective provider tunnels are signaled using Type 3 Selective-PMSI (S-PMSI) autodiscovery routes. See Figure 1 and Table 2 for details. The sender PE router sends a Type 3 route to signal that it is sending traffic for a particular (C-S, C-G) flow using an S-PMSI provider tunnel.

Figure 1: S-PMSI Autodiscovery Route Type Multicast (MCAST)-VPN Network Layer Reachability Information (NLRI) Format

Table 2: S-PMSI Autodiscovery Route Type Format Descriptions

Field	Description
Route Distinguisher	Set to the route distinguisher configured on the router originating this route.
Multicast Source Length	Set to 32 for IPv4 and to 128 for IPv6 C-S IP addresses.
Multicast Source	Set to the C-S IP address.
Multicast Group Length	Set to 32 for IPv4 and to 128 for IPv6 C-G addresses.
Multicast Group	Set to the C-G address.

The S-PMSI autodiscovery (Type 3) route carries a PMSI attribute similar to the PMSI attribute carried with intra-AS autodiscovery (Type 1) routes. The Flags field of the PMSI attribute carried by the S-PMSI autodiscovery route is set to the leaf information required. This flag signals receiver PE routers to originate a Type 4 leaf autodiscovery route (Figure 2) to join the selective provider tunnel if they have active receivers. See Table 3 for details of leaf autodiscovery route type MCAST-VPN NLRI format descriptions.

Figure 2: Leaf Autodiscovery Route Type MCAST-VPN NLRI Format

Table 3: Leaf Autodiscovery Route Type MCAST-VPN NLRI Format Descriptions

Field	Description
Route Key	Contains the original Type 3 route received.
Originating Router’s IP Address	Set to the IP address of the PE router originating the leaf autodiscovery route This is typically the primary loopback address.