https://networklessons.com/multicast/multicast-routing/
IP多播有两种模式，密集模式和稀疏模式：
- Dense Mode
- Sparse Mode
密集模式下，每个路由器收到多播包之后，发往除接收接口之外的所有其它接口，类似，洪泛。
稀疏模式，待补充。。。
密集模式下，很容易产生路由环路，通过采用返向路径转发RPF (Reverse Path Forwarding)，来实现路由环路的避免，流程如下：
Above we have R1 which receives a multicast packet which is flooded on all interfaces except the interface that connects to the video server. I’m only showing the packet that is flooded towards R3 here:
- R1 floods the packet to R3.
- R3 floods the packet to R2.
- R2 floods it back to R1.
We now have a multicast routing loop. We can prevent this by implementing the RPF check:
When a router receives a multicast packet on an interface, it looks at the source IP address and does two checks:
- Do we have an entry that matches the source address in the unicast routing table?
- If so, what interface do we use to reach that source address?
When the multicast packet is received on the interface that matches the information from the unicast routing table, it passes the RPF check and we accept the packet. When it fails the RPF check, we drop the packet.