RPF Checks on PE Devices
If multicast VPN is not configured, network devices select an optimal route to a multicast source from their routing table as the RPF route. The RPF route contains the RPF interface (outbound interface of the unicast route) and RPF neighbor (next hop of the unicast route). The RPF neighbor information is used to construct PIM Join/Prune messages, and the RFP interface information is used for RPF checks. During an RPF check, a multicast device checks whether the packet is received from the RPF interface. If so, the packet passes the RPF check and can be forwarded through the outbound interface. If not, the packet fails the RPF check and is dropped. For details about RPF checks, see RPF Check.
After multicast VPN is configured, CE and P devices cannot detect VPNs. The RPF check mechanism used on them remains unchanged. That is, the RPF interface is the outbound interface of a unicast route to the source address, and the RPF neighbor is the next hop of the unicast route.
For PE devices, the RPF check mechanism used in the public network instance remains unchanged after multicast VPN configuration. For VPN instances, RPF information needs to be redefined based on the outbound interface of a unicast route, so that VPN packets can pass RPF checks and multicast distribution trees (MDTs) can be established for VPN instances across the public network.
VPN Interface as the Outbound Interface of the Unicast Route
If the outbound interface of the unicast route to a multicast source is a VPN interface, RPF information remains the same after multicast VPN is configured.
As shown in Figure 1, PE1 has a unicast route to the multicast source 192.168.1.1/24, with VPN interface PE1-if1 as the outbound interface and the IP address of CE1-if1 as the next hop address. In this case, PE1-if1 is the RPF interface and CE1 is the RPF neighbor. When PE1 receives packets from 192.168.1.1 on PE1-if1, the packets pass the RPF check and are forwarded to the outbound interface.
Public Network Interface as the Outbound Interface of the Unicast Route
When the outbound interface of the unicast route to a multicast source is a public network interface, the RPF interface is the MTI on the local PE. A remote PE meeting the following conditions is the RPF neighbor:
- It is the next hop of the BGP route from the local PE to the multicast source.
- It is the PIM neighbor of the local PE.
A neighbor relationship is established between PE devices through MTIs. The PIM neighbor address is the MTI IP address on the peer PE. Therefore, a PE becomes the RPF neighbor only when its MTI IP address is the same as that used for establishing IBGP connections.
As shown in Figure 2, PE2 has a route to the multicast source 192.168.1.1/24, with the outbound interface as public network interface PE2-if2, and the next hop as 10.1.1.1/24 (PE1). PE1 is next hop of the BGP route from PE2 to the multicast source and the PIM neighbor of PE2. Therefore, PE1 is the RPF neighbor of PE2. The MTI on PE2 is the RPF interface. When PE2 receives packets from 192.168.1.1 on the MTI, the packets pass the RPF check and are forwarded to the outbound interface.
