A RP is the focal point for multicast traffic. Traffic is forwarded to the RP from multicast sources. The RP then forwards traffic to multicast receivers.
Used with PIMSM Auto-RP and version 2. If the RPs fail, the router reverts to dense mode.
Two or more RPs are configured with the same IP address. The IP addresses of the RPs are advertised using a unicast IP routing protocol. Each multicast router chooses the closest RP. If an RP fails, the routers switch to the next nearest RP after the unicast IP routing protocol converges. The MSDP is used between RPs to exchange active multicast source information.
Multicast forwarding decisions are based on the entries in the unicast IP routing table. Multicast is not dependent on how the unicast IP routing table was built; you can use any dynamic interior routing protocol, static routes, or a combination of the two.
The low order 32 bits of the IP address determine the multicast Ethernet address. The first four bits are always 1 1 1 0 and the next five bits can be anything. Therefore, the IP multicast addresses that map to the multicast Ethernet address of 01 00 5E 00 40 0C are
1110 0000 0000 0000 0100 0000 1100 = 126.96.36.199
1110 0000 1000 0000 0100 0000 1100 = 188.8.131.52
1110 0001 0000 0000 0100 0000 1100 = 184.108.40.206
The base Ethernet multicast address is 01 00 5E 00 00 00. The first byte of the IP multicast address is not used. If the second byte is greater than 127, subtract 128, giving a value of 0. The third and fourth bytes of the IP address are used as is after converting to hex. Their values, in hexadecimal, are 40 and 0C. So the Ethernet multicast address for the IP multicast address 220.127.116.11 is 01 00 5E 00 40 0C.
Dense mode multicast assumes all multicast neighbors want to receive all multicast traffic unless the neighbors have specifically pruned the traffic. Sparse mode multicast assumes multicast neighbors do not want to receive multicast traffic unless they have asked for it. Dense mode uses source-based delivery trees while sparse mode uses shared delivery trees where traffic is first sent to an RP.