MDT-based MVPN inter-AS option B
In MVPN inter-AS option B, RPF vector and BGP connector are introduced:
RPF vector—Attribute encapsulated in a PIM join message. It is the next hop of BGP MDT route from the local PE to the remote PE. Typically, it is the ASBR in the local AS.
When a device receives the join message with the RPF vector, it first checks whether the RPF vector is its own IP address. If so, the device removes the RPF vector, and sends the message to its upstream neighbor according to the route to the remote PE. Otherwise, it keeps the RPF vector, looks up the route to the RPF vector, and sends the message to the next hop of the route. In this way, the PIM message can be forwarded across the ASs and an MDT is established.
BGP connector—Attribute shared by BGP peers when they exchange IPv4 VPN routes. It is the IP address of the remote PE.
The local PE fills the upstream neighbor address field with the BGP connector in a join message. This ensures that the message can pass the RPF check on the remote PE after it travels along the MT.
To implement MVPN inter-AS option B, only one MVPN needs to be established for the two ASs. VPN multicast data is transmitted between different ASs on the public network within this MVPN.
As shown in Figure 75:
A VPN network involves AS 1 and AS 2.
PE 3 and PE 4 are the ASBRs for AS 1 and AS 2, respectively.
PE 3 and PE 4 are interconnected through MP-EBGP and treat each other as a P device.
PE 3 and PE 4 advertise VPN-IPv4 routes to each other through MP-EBGP.
An MT is established between PE 1 and PE 2 for delivering VPN multicast traffic across the ASs.
Figure 75: MVPN inter-AS option B
The establishment of the MDT on the public network is as follows:
PE 1 originates a PIM join message to join the SPT rooted at PE 2. In the join message, the upstream neighbor address is the IP address of PE 2 (the BGP connector). The RPF vector attribute is the IP address of PE 3. PE 1 encapsulates the join message as a public network packet and forwards it through the MTI.
P 1 determines that the RPF vector is not an IP address of its own. It looks up the routing table for a route to PE 3, and forwards the packet to PE 3.
PE 3 removes the RPF vector because the RPF vector is its own IP address. It fails to find a BGP MDT route to PE 2, so it encapsulates a new RPF vector (IP address of PE 4) in the packet and forwards it to PE 4.
PE 4 removes the RPF vector because the RPF vector is its own IP address. It has a local route to PE 2, so it forwards the packet to P 2, which is the next hop of the route to PE 2.
P 2 sends the packet to PE 2.
PE 2 receives the packet on the MTI and decapsulates the packet. The receiving interface is the RPF interface of the RPF route back to PE 1 for the join message, and the join message passes the RPF check. The SPT from PE 1 to PE 2 is established.
When PE 1 joins the SPT rooted at PE 1, PE 2 also initiates a join process to the SPT rooted at PE 1. A MDT is established when the two SPTs are finished.