Understanding Next-Generation MVPN Control Plane

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

Usage	Type	Name	Description
Membership autodiscovery routes for inclusive provider tunnels	1	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.
	2	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	3	S-PMSI AD route	Originated by a sender router. Used for initiating a selective provider tunnel for a particular (C-S, C-G).
	4	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	5	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	6	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.
	7	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.

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

Field	Description
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.

Table 4: PMSI Tunnel Attribute Format Descriptions

Field	Description
Flags	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[10.1.1.1:0:6574:10.1.1.1]

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.

Converting MVPN Type 5 Routes to MSDP SA

You can convert next-generation multicast virtual private network (MVPN) Type 5 routes to Multicast Source Discovery Protocol (MSDP) source active (SA) routes. This conversion helps in reducing the number of MSDP sessions running between VPN customer rendezvous points (C-RPs). For example, instead of having MSDP running among all C-RPs in a deployment, the C-RPs could instead run their MSDP sessions on a single PE router configured for multiple MSDP peers. The PE router, acting as a C-RP device, receives MVPN SA Type 5 routes from the RP-PE or the source PE router, converts those routes to MSDP, and advertises the MSDP routes to its MSDP peers.

MVPN Type 5 SA routes are added to the MVPN table. These MVPN Type 5 SA routes also include a new Extended Community (EC) with the IPv4 address of the RP where the MVPN SA was generated. The Type 5 route's source and EC are additionally added to the MSDP table. Stale routes, including the EC, are removed from the MSDP table after the MVPN Type 5 SA route is cleared from the MVPN table.

Conversion of MVPN Type 5 routes to MSDP SA is defined in RFC 9081. Previously, Junos OS only supported conversion from MSDP SA routes to MVPN Type 5.

Note:

MVPN Type 5 routes to MSDP SA conversion works in spt-only mode. rpt-spt mode is not supported.

The Extended Community is IPv4 only. IPv6 is not supported, as MSDP is IPv4 only.

• Benefits of converting MVPN Type 5 routes to MSDP SA
• Enabling the conversion of MVPN Type 5 routes to MSDP SA
• Verifying the conversion of MVPN Type 5 routes to MSDP SA