BGP-MVPN Source Active Route Enhancement

Background Section "14. Supporting PIM-SM without Inter-Site Shared C-Trees" of specifies one way of supporting Any-Source Multicast (ASM) for Multicast Virtual Private VPN (MVPN), referred to as SPT-only mode in this document. Consider the following topology:

When a receiver connected to the Last Hop Router (LHR) CE3 joins an ASM group, CE3 sends a (*, G) Protocol Independent Multicast (PIM) Join Messages to PE3, which is the upstream router towards the Customer-Rendezvous-Point (C-RP) on PE2. PE3 does not originate a (C-*, C-G) C-Multicast BGP route (an equivalent of the PIM (*, G) join message). Rather, it only originates (C-S, C-G) C-multicast BGP routes (equivalent of PIM (S, G) join messages) when different sources for the group G are discovered. When the source starts sending multicast traffic for the ASM group G, the First Hop Router (FHR) CE1 sends a PIM Register message to the C-RP. PE2 then originates a (C-S, C-G) Source Active (SA) BGP route to be imported by all PEs for the VPN. As a result, PE3 originates a (C-S, C-G) C-multicast route targeted at PE1 - the Upstream Multicast Hop (UMH) for the C-S. PE1 then sends a corresponding (S, G) PIM join message towards CE1. specifies the following situations where a PE originates SA routes:

In the above example, only PE2 will originate the SA routes. After the signaling is done and traffic starts flowing, the LHR CE3 has the option of sending a PIM (S, G) join message but it may choose not to. If PE3 does not have the (S, G) join from CE3, and the SA route from PE2 is withdrawn/invalidated for whatever reason (e.g. the BGP session with PE2 goes down), PE3 will withdraw its (C-S, C-G) C-Multicast route and PE1 will stop sending traffic. This PE2-related failure should not have caused traffic loss since it is not in the path at all. This can be avoided if PE1 also originates the (C-S, C-G) SA route when it receives the (C-S, C-G) C-multicast route. When PE1 does not receive the (C-S, C-G) traffic for a while, it withdraws its corresponding SA route, even though it still has the (C-S, C-G) C-multicast route (otherwise the SA route and C-multicast route will stay forever). As long as there is any (C-S, C-G) SA route from any PE, PE3 will originate its (C-S, C-G) C-Multicast Route targeting at the UMH PE of its own choice. In the above example, while PE2 first originates the SA route, PE3 still targets its C-Multicast route at PE1. When PE2's SA route is invalidated/withdrawn, PE3 will keep its (C-S, C-G) C-Multicast route if the SA route from PE1 is present.

Specification This section specifies an optional enhancement to the origination of SA routes in the SPT-only mode for supporting ASM. Any PE that discovers a (C-S, C-G) flow on its PE-CE interfaces MUST originate a corresponding (C-S, C-G) SA route, and MUST withdraw the route when the flow is deemed no longer active. Besides the methods of discovering a flow described in the section 14 of (as quoted earlier), a PE MAY use the its (C-S, C-G) state for this purpose if the Reverse Path Forwarding (RPF) interface is a PE-CE interface. The state may have been triggered by the arrival of traffic on the PE-CE interface or by a corresponding (C-S, C-G) C-Multicast route. when the (C-S, C-G) state is first created, the PE MAY immediately originate a corresponding Source Active route without waiting for the arrival of the traffic. When it stops receiving traffic for over the PIM Keepalive_Period or if the (C-S, C-G) state is deleted, it MUST withdraws the SA route. Note that while the Keepalive_Period value is used here, it does not require the Keepalive Timer mechanism in .