Configuring SPT-Only Mode for Multiprotocol BGP-Based Multicast VPNs
For MBGP MVPNs (also referred to as next-generation Layer 3 multicast VPNs), the default mode of operation is shortest path tree only (SPT-only) mode. In SPT-only mode, the active multicast sources are learned through multicast VPN source-active routes. This mode of operation is described in section 14 of the BGP-MVPN draft (draft-ietf-l3vpn-2547bis-mcast-bgp-00.txt).
In contrast to SPT-only mode, rendezvous point tree (RPT)-SPT mode (also known as shared-tree data distribution) supports the native PIM model of transmitting (*,G) messages from the receiver to the RP for intersite shared-tree join messages.
In SPT-only mode, when a PE router receives a (*, C-G) join message, the router looks for an active source transmitting data to the customer group. If the PE router has a source-active route for the customer group, the router creates a source tree customer multicast route and sends the route to the PE router connected to the VPN site with the source. The source is determined by MVPN's single-forwarder election. When a receiver sends a (*,G) join message in a VPN site, the (*,G) join message only travels as far as the PE router. After the join message is converted to a type 6 multicast route, which is equivalent to a (S,G) join message, the route is installed with the no-advertise community setting.
The MVPN single-forwarder election follows the rule documented in section 9.1.1 of the BGP-MVPN draft (draft-ietf-l3vpn-2547bis-mcast-bgp-00.txt). The single-forwarder election winner is based on the following rules:
If the active unicast route to the source is through the interface, then this route is used to determine the upstream multicast hop (UMH).
If the active unicast route to the source is a VPN route, MVPN selects the UMH based on the highest IP address in the route import community for the VPN routes, and the local primary loopback address for local VRF routes.
Single-forwarder election guarantees selection of a unique forwarder for a given customer source (C-S). The upstream PE router might differ for the source tree and the shared tree because the election is based on the customer source and C-RP, respectively. Although the single-forwarder election is sufficient for SPT-only mode, the alternative RPT-SPT mode involves procedures to prevent duplicate traffic from being sent on the shared tree and the source tree. These procedures might require administrator-configured parameters to reduce duplicate traffic and reduce null routes during RPT to SPT switch and the reverse.
In SPT-only mode, when a source is active, PIM creates a register state for the source both on the DR and on the C-RP (or on a PE router that is running Multicast Source Discovery Protocol [MSDP] between itself and the C-RP). After the register states are created, MVPN creates a source-active route. These type 5 source-active routes are installed on all PE routers. When the egress PE router with the (*,G) join message receives the source-active route, it has two routes that it can combine to produce the (S,G) multicast route. The type 6 route informs the PE router that a receiver is interested in group G. The source active route informs the PE router that a source S is transmitting data to group G. MVPN combines this information to produce a multicast join message and advertises this to the ingress PE router, as determined by the single-forwarder election.
For some service providers, the SPT-only implementation is not ideal because it creates a restriction on C-RP configuration. For a PE router to create customer multicast routes from (*, C-G) join messages, the router must learn about active sources through MVPN type 5 source-active routes. These source-active routes can be originated only by a PE router. This means that a PE router in the MVPN must learn about all PIM register messages sent to the RP, which is possible only in the following cases:
The C-RP is colocated on one of the PEs in the MVPN.
MSDP is run between the C-RP and the VRF instance on one of the PE routers in the MVPN.
If this restriction is not acceptable, providers can use RPT-SPT mode instead of the default SPT-only mode. However, because SPT-only mode does not transmit (*,G) routes between VPN sites, SPT-only mode has the following advantages over RPT-SPT mode:
Simplified operations by exchanging and processing only source-tree customer multicast routes among PE routers
Simplified operations by eliminating the need for the service provider to suppress MVPN transient duplicates during the switch from RPT to SPT
Less control plane overhead in the service provider space by limiting the type of customer multicast routes exchanged, which results in more scalable deployments
More stable traffic patterns in the backbone without the traffic shifts involved in the RPT-SPT mode
Easier maintenance in the service provider space due to less state information