Understanding Next-Generation MVPN Control Plane


The BGP next-generation multicast virtual private network (MVPN) control plane, as specified in Internet draft draft-ietf-l3vpn-2547bis-mcast-10.txt and Internet draft draft-ietf-l3vpn-2547bis-mcast-bgp-08.txt, distributes all the necessary information to enable end-to-end C-multicast routing exchange via BGP. The main tasks of the control plane (Table 1) include MVPN autodiscovery, distribution of provider tunnel information, and PE-PE C-multicast route exchange.

Table 1: Next-Generation MVPN Control Plane Tasks

Control Plane Task


MVPN autodiscovery

A provider edge (PE) router discovers the identity of the other PE routers that participate in the same MVPN.

Distribution of provider tunnel information

A sender PE router advertises the type and identifier of the provider tunnel that it will use to transmit VPN multicast packets.

PE-PE C-Multicast route exchange

A receiver PE router propagates C-multicast join messages (C-joins) received over its VPN interface toward the VPN multicast sources.

BGP MCAST-VPN Address Family and Route Types

Internet draft draft-ietf-l3vpn-2547bis-mcast-bgp-08.txt introduced a BGP address family called MCAST-VPN for supporting next-generation MVPN control plane operations. The new address family is assigned the subsequent address family identifier (SAFI) of 5 by the Internet Assigned Numbers Authority (IANA).

A PE router that participates in a BGP-based next-generation MVPN network is required to send a BGP update message that contains MCAST-VPN network layer reachability information (NLRI). An MCAST-VPN NLRI contains route type, length, and variable fields. The value of each variable field depends on the route type.

Seven types of next-generation MVPN BGP routes (also referred to as routes in this topic) are specified (Table 2). The first five route types are called autodiscovery MVPN routes. This topic also refers to Type 1-5 routes as non-C-multicast MVPN routes. Type 6 and Type 7 routes are called C-multicast MVPN routes.

Table 2: Next-generation MVPN BGP Route Types





Membership autodiscovery routes for inclusive provider tunnels


Intra autonomous system (intra-AS) I-PMSI autodiscovery route

  • Originated by all next-generation MVPN PE routers.

  • Used for advertising and learning intra autonomous system (intra-AS) MVPN membership information.


Inter-AS I-PMSI AD route

  • Originated by next-generation MVPN ASBR routers.

  • Used for advertising and learning inter-AS MVPN membership information.

Autodiscovery routes for selective provider tunnels


S-PMSI AD route

  • Originated by a sender router.

  • Used for initiating a selective provider tunnel for a particular (C-S, C-G).


Leaf AD route

  • Originated by receiver PE routers in response to receiving a Type 3 route.

  • Used by a sender PE router to discover the leaves of a selective provider tunnel.

  • Also used for inter-AS operations that are not covered in this topic.

VPN multicast source discovery routes


Source active AD route

  • Originated by the PE router that discovers an active VPN multicast source.

  • Used by PE routers to learn the identity of active VPN multicast sources.

C-Multicast routes


Shared tree join route

  • Originated by receiver PE routers.

  • Originated when a PE router receives a shared tree C-join (C-*, C-G) through its PE-CE interface.


Source tree join route

  • Originated by receiver PE routers.

  • Originated when a PE router receives a source tree C-join (C-S, C-G) or originated by the PE router that already has a Type 6 route and receives a Type 5 route.

Intra-AS MVPN Membership Discovery (Type 1 Routes)

All next-generation MVPN PE routers create and advertise a Type 1 intra-AS autodiscovery route (Figure 1) for each MVPN to which they are connected. Table 3 describes the format of each MVPN Type 1 intra-AS autodiscovery route.

Figure 1: Intra-AS I-PMSI AD Route Type MCAST-VPN NLRI Format
Intra-AS I-PMSI AD Route Type MCAST-VPN NLRI Format

Table 3: Type 1 Intra-AS Autodiscovery Route MVPN Format Descriptions



Route Distinguisher

Set to the route distinguisher configured for the VPN.

Originating Router’s IP Address

Set to the IP address of the router originating this route. The address is typically the primary loopback address of the PE router.

Inter-AS MVPN Membership Discovery (Type 2 Routes)

Type 2 routes are used for membership discovery between PE routers that belong to different autonomous systems (ASs). Their use is not covered in this topic.

Selective Provider Tunnels (Type 3 and Type 4 Routes)

A sender PE router that initiates a selective provider tunnel is required to originate a Type 3 intra-AS S-PMSI autodiscovery route with the appropriate PMSI attribute.

A receiver PE router responds to a Type 3 route by originating a Type 4 leaf autodiscovery route if it has local receivers interested in the traffic transmitted on the selective provider tunnel. Type 4 routes inform the sender PE router of the leaf PE routers.

Source Active Autodiscovery Routes (Type 5 Routes)

Type 5 routes carry information about active VPN sources and the groups to which they are transmitting data. These routes can be generated by any PE router that becomes aware of an active source. Type 5 routes apply only for PIM-SM (ASM) when intersite source-tree-only mode is being used.

C-Multicast Route Exchange (Type 6 and Type 7 Routes)

The C-multicast route exchange between PE routers refers to the propagation of C-joins from receiver PE routers to the sender PE routers.

In a next-generation MVPN, C-joins are translated into (or encoded as) BGP C-multicast MVPN routes and advertised via the BGP MCAST-VPN address family toward the sender PE routers.

Two types of C-multicast MVPN routes are specified:

  • Type 6 C-multicast routes are used in representing information contained in a shared tree (C-*, C-G) join.

  • Type 7 C-multicast routes are used in representing information contained in a source tree (C-S, C-G) join.

PMSI Attribute

The provider multicast service interface (PMSI) attribute (Figure 2) carries information about the provider tunnel. In a next-generation MVPN network, the sender PE router sets up the provider tunnel, and therefore is responsible for originating the PMSI attribute. The PMSI attribute can be attached to Type 1, Type 2, or Type 3 routes. Table 4 describes each PMSI attribute format.

Figure 2: PMSI Tunnel Attribute Format
PMSI Tunnel Attribute Format

Table 4: PMSI Tunnel Attribute Format Descriptions




Currently has only one flag specified: Leaf Information Required. This flag is used for S-PMSI provider tunnel setup.

Tunnel Type

Identifies the tunnel technology used by the sender. Currently there are seven types of tunnels supported.

MPLS Label

Used when the sender PE router allocates the MPLS labels (also called upstream label allocation). This technique is described in RFC 5331 and is outside the scope of this topic.

Tunnel Identifier

Uniquely identifies the tunnel. Its value depends on the value set in the tunnel type field.

For example, Router PE1 originates the following PMSI attribute:

PMSI: Flags 0:RSVP-TE:label[0:0:0]:Session_13[]

VRF Route Import and Source AS Extended Communities

Two extended communities are specified to support next-generation MVPNs: source AS (src-as) and VRF route import extended communities.

The source AS extended community is an AS-specific extended community that identifies the AS from which a route originates. This community is mostly used for inter-AS operations, which is not covered in this topic.

The VPN routing and forwarding (VRF) route import extended community is an IP-address-specific extended community that is used for importing C-multicast routes in the VRF table of the active sender PE router to which the source is attached.

Each PE router creates a unique route target import and src-as community for each VPN and attaches them to the VPN-IPv4 routes.