From CCIE Study Wiki

There are several different methods that routers can learn the address of a rendezvous point in a PIM-SM network

Theory

A PIM-SM router has three ways to learn the address of a RP:

• RP can be statically configured on all routers with the ip pim rp-address command.
• The Auto-RP protocol is a Cisco-proprietary protocol that can be used to designate the RP and advertise its IP address to other routers
• The BootStrap Router (BSR) protocol can designate and advertise the RP

You can also have redundant RPs in a network. Bootstrap Router Or BSR can provide this, as can Anycast RP using Multicast Source Discovery Protocol (MSDP).

Auto-RP - When using Auto-RP, the RP will send a RP-Announce message to 224.0.1.39 stating that the router is a RP and announcing which multicast groups it is the RP for. This allows you to load-balance groups between different RP's. These RP-Announce messages are sent every minute.

A router called the mapping agent (often the same router) learns all of the RPs and the multicast groups that each one supports. The mapping agent then sends another message called RP-Discovery that identifies the RP for each range of multicast group addresses. This message goes to 224.0.1.40, and is the actual message that informs routers what they should use for RPs for any given group.

• If multiple RPs are advertising for the same group, the mapping agent selects the router with the highest IP address as the RP for the group.
• All routers with PIM-SM and Auto-RP configured will be listening on 224.0.1.40, and maintaining any group-to-RP mapping info in their cache.

To support auto-RP, Cisco has implemented another version of PIM called sparse-dense mode.

• This overcomes the issue where that a router configured with PIM-SM and Auto-RP needs to send a Join message towards the RP to start to receive Auto-RP messages, but they don't know where the RP is located yet.
• In PIM sparse-dense mode, a router uses PIM-DM rules when it does not know the location of an RP, and SM rules when it does know the location of a RP.

BootStrap Router (BSR) - works similar in theory to Auto-RP: sends a message to another router, which collects the group-to-RP info and distributes it to other routers.

One router is configured as the BSR, which is similar to the mapping agent in Auto-RP. It differs in that:

• The BSR does not pick the best RP for each group - all group-to-mapping info is sent out to all PIM routers using bootstrap messages
• Each PIM router pick the best RP themselves
• BSR sends out bootstrap messages via 224.0.0.13
• This flooding does NOT require the routers to have a known RP or support dense mode

If multiple BSRs are configured each candidate BSR will include the priority of the BSR and its IP address. The highest priority (or highest IP in a tie) BSR wins. This "preferred" BSR continues to send bootstrap messages, which the backup BSRs monitor for an outage.

Anycast RP with MSDP - Anycast RP is a method, in use with Auto-RP or BSR, that multiple RPs can act as RP for the same group for redundancy and load sharing.

When using anycast RP, each RP will have the same (loopback) IP address. These addresses are advertised within an IGP, with the end result being that all RPs are viewed by the other routers as a single RP. Any packets sent to the RP are routed via the IGP to the closest RP.

With anycast RP, multiple RPs share the load for a single multicast group, and recovery after a failed RP happens as fast as IGP convergence takes to point to another RP.

There is a problem with anycast RP that is solved through the use of MSDP.

• A multicast source will send packets to one of the RPs. Without some way for RPs to share information about known sources, all of the RPs would not get multicasts from every source.
• MSDP allows RPs to send messages to each other, revealing the IP address of each (S,G) to all of the RP's, and allowing the RP to join the SPT of the source it was missing before.

Commands

• ip pim sparse-dense mode (interface)

Default Settings

• RP-Announce messages sent out every 60 secs by a RP

Verification

• Nothing listed yet

Troubleshooting, Tips, and Tricks

• Router Ric explains the answer to a multicast problem that can be accomplished through the use of BSR.

• Auto-RP mapping agents should generally be configured inside the central site network in order to prevent any issues with Auto-RP discovery.

Online Resources

• Petr from Internetwork Expert gives an in-depth explanation of the BSR protocol.

• CCIE the Beginning has links to awesome animations of PIM sparse mode with auto-RP and PIM sparse mode with BSR. There is also an example of configuring auto-rp.

• How to choose between auto-RP and BSR from Tassos at CCIE in 3 Months.

• A configuration example of auto-RP filtering and using RPs for load balancing and redundancy from Router Ric.

• The Human Modem has a series of articles on multicast RP's:
  • PIM sparse mode - static RP
  • PIM sparse mode - auto RP
  • PIM sparse mode - BSR

• Bradley of Community String covers manual RP selection and Auto RP as well as BSR and anycast MSDP.

• Examining the RP-announce filter command from Bryan of IP Expert.

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