Auto-RP automatically distributes information to routers as to what the RP address is for various multicast groups. It simplifies use of multiple RP's for different multicast group ranges. It avoids manual configuration inconsistencies, and allows for multiple RP's acting as backups to each other. Cisco routers automatically listen for this information.

Auto-RP relies on a router designated as RP mapping agent. Potential RP's announce themselves to the mapping agent, and it resolves any conflicts. The mapping agent then sends out the multicast group-RP mapping information to the other routers.

How does it does this? It uses multicast to send the mapping information to the other routers! The specific groups used are and .40. The first (.39) is used to advertise, the second (.40) is used for discovery. Of course, there's a chicken and egg problem there: how can you send out multicast information via multicast if the Auto-RP information is needed to make PIM-SM work in the first place?

Generally Auto-RP is used with sparse-dense mode, since then the Auto-RP information can be propagated in dense mode. If your routers are configured with pure sparse-mode on the interfaces, then you can shift to sparse-dense-mode. The other choice with PIM-SM only interfaces is to configure static RP addresses for the Auto-RP multicast groups (the multicast groups used by Auto-RP itself to communicate). That way, the static info gets the Auto-RP multicasts distributed in sparse mode, and then the Auto-RP mapping information allows the other multicast groups to be joined. By the way, you do not need to statically specify a group range for the Auto-RP multicast groups, since normally Auto-RP information takes priority over statically configured information. Thus group mappings advertised via Auto-RP will direct Joins to the correct RP, while the lack of this information for the Auto-RP groups means the statically configured RP for Auto-RP will remain in effect.

Routers that are RP's are configured with the global configuration command:

ip pim send-rp-announce type number scope ttl-value [group-list access-list] [interval seconds] [bidir]

The type number argument is the name of an interface providing the address for the RP. Scope is the TTL of the announcement (which limits how many router hops it can traverse). You specify which multicast groups the router is RP for with the access-list.

For example:
ip pim send-rp-announce loopback0 scope 16 group-list 10
access-list 10 permit

By default, such routers advertise themselves every 60 seconds to multicast group They advertise their address, also the range of groups they are RP for. The mapping agents receive this information. They select the highest candidate RP address as RP for each group or range advertised. The mapping agents advertise this, by default every 60 seconds or when changes occur, to multicast group You do have to configure mapping agents (so they know they're the mapping agent):

ip pim send-rp-discovery scope ttl-value

The scope is how many hops the advertisements can take. This allows you to have different mapping agents, each responsible for part of the network. (Some other configuration may also be required to optimize this.)

Because the Auto-RP mapping agents use the highest RP for each group or range, you can have redundant RP's. If the one with the highest address fails, the next one will take over (after the cache hold time expires). If you have redundant Auto-RP mapping agents, as long as they advertise the same information, there is no problem. You do need to make sure candidate RP's use a large enough scope to reach all the mapping agents.

If you use administrative scoping (ttl settings), generally you set them on the large side, to make sure the reach every router within the local domain (part of the network). Think of the multicast advertisements as a wave, with scope being the height of the wave. You need to make sure the wave is big enough to reach the fringes of the domain. If you need to keep them from "spilling over" into another part of the network, you can use a boundary command:

interface serial 0
ip multicast boundary 10
access-list 10 deny
access-list 10 permit

This stops specified multicasts from crossing the boundary. You can also use a TTL threshold:

interface serial 0
ip multicast ttl-threshold ttl-value

Only the packets with TTL greater than the threshold are forwarded out the interface.
Routers will act as RP if they receive Join or Prune messages. You can configure your routers to only accept prunes and joins in accord with the Auto-RP mapping information, with:

ip pim accept-rp auto-rp [access-list]

The access list can be used to control which groups this applies to. You could also statically configure

ip pim accept-rp rp-address [access-list]

for static RP's, but that gets painful to maintain.
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