Core Issue
Protocol Independent Multicast (PIM) sparse mode is the most scalable protocol for IP multicasting. This relies on an explicit join model to forward multicast traffic. The multicast sources and receivers register with a device functioning as a Rendezvous Point (RP) to form the multicast distribution tree. Every router in the sparse mode network has to know the address of the device functioning as RP.
Resolution
To configure an RP, choose one of these options:
- To configure a static RP address, issue the ip pim rp-address command. This method is not scalable and does not provide redundancy for RP. It is recommended only in very small, simple networks. All the routers are configured to operate in sparse mode by issuing the ip pim sparse-mode command under the interfaces.
- To distribute the RP information to all the routers, use the Cisco proprietary Auto-RP mechanism. This also provides redundancy. If an RP is known, Auto-RP uses sparse-dense mode, which allows a group to be treated in sparse mode. If an RP is not known, the group defaults to dense mode. RP information is conveyed to the routers by the Auto-RP mechanism. This mechanism uses two well-known groups operating in dense mode: 224.0.1.39 and 224.0.1.40.
Devices functioning as mapping agents listen to the 224.0.1.39 group address to announce candidate RPs. This is done by issuing the ip pim send-rp-announce command. The mapping agents issued with the ip pim send-rp-discovery command listen to the 224.0.1.39 address, choose the RP for the group based on the one with the highest RP address, and advertises it to the 224.0.1.40 group address to which all other routers listen. The advantage of this method is that any change to the RP designation needs to be configured only on the RP routers, and not on the leaf routers, as in the static configuration. All the routers need to be configured to operate in both sparse and dense mode. Issue the ip pim sparse-dense-mode command under the interfaces.
- Use the standards based Cisco Bootstrap Router (BSR) mechanism available in PIM version 2. This method also provides the same benefit of Auto-RP. Devices configured as candidate BSRs by issuing the ip pim bsr-candidate command are equivalents of mapping agents in Auto-RP. These devices announce themselves to other routers using the 224.0.0.13 group address. Since a message sent to this address is forwarded hop-by-hop throughout the network, all the routers learn about the candidate BSRs and choose one of them as the BSR based on the highest priority or IP address.
Similar to Auto-RP, candidate RPs issued with the ip pim rp-candidate command advertise themselves to devices functioning as a BSR through the unicast mechanism. The RP set information is then advertised by the BSR to the 224.0.0.13 group address for other routers to learn about the RPs. The difference from Auto-RP is that the interfaces can be configured to operate only in sparse mode by issuing the ip pim sparse-mode command, since BSR messages are flooded hop-by-hop. Another difference is that each router receiving the RP set information from the BSR uses a hash algorithm to select the RP for a group. In Auto-RP mechanism, the mapping agent selects and advertises the group to RP mapping to leaf routers.
For more information on configuring an RP for PIM sparse mode, refer to these documents:
