Multicast AutoRP

Auto-RP is a Cisco proprietary mechanism. AutoRP defines three router types (just like BSR):

Standard - does not participate in RP advertisements or Mapping but forwards multicast traffic based on received RP information.
Mapping Agents - The router which makes the decision about what is the current active RP
RP - The router which is part of the multicast forwarding tree


Candidate RPs advertize their willingness to be an RP via "RP-announcement" messages. These messages are periodically sent to a reserved well-known group 224.0.1.39 (CISCO-RP-ANNOUNCE).

RP mapping agents join group 224.0.1.39 and map the RPs to the associated groups. The RP mapping agents advertise the authoritative RP-mappings to another well-known group address 224.0.1.40 (CISCO-RP-DISCOVERY). All PIM routers join 224.0.1.40 and store the RP-mappings in their private cache. 

For IPv4: AutoRP is enabled as soon as you enable ip multicast-routing and turn on PIM. You can check with with the following two commands:

R1#show ip mroute
IP Multicast Routing Table

(*, 224.0.1.40), 00:02:44/00:02:58, RP 0.0.0.0, flags: DCL
  Incoming interface: Null, RPF nbr 0.0.0.0
  Outgoing interface list:
    Ethernet0/0, Forward/Sparse, 00:02:42/00:00:17

R1#show ip pim autorp
AutoRP Information:
  AutoRP is enabled.

PIM AutoRP Statistics: Sent/Received
  RP Announce: 0/0, RP Discovery: 0/0

By default the grouts 224.0.1.39 and 224.0.1.40 are multicasted in dense mode. This mean you will need to enable ip pim sparse-dense mode. This has a security issue that you cannot control which other groups are using dense mode.

To reduce this risk you can configure ip pim spase-mode and the use the global command ip pim autorp listner to cause IP multicast traffic for the two Auto-RP groups 224.0.1.39 and 224.0.1.40 to be dense mode flooded across interfaces operating in PIM sparse mode.

For IPv6: Auto-RP is not supported for IPv6 Multicast.


You can configure a set of routers to be the Mapping Agent as follows:


IPv4
ip pim [vrf vrf-name] send-rp-discovery [interface-type interface-number] scope ttl-value [interval seconds] 

If there are multiple mapping agents, then all of them operate independently as they arrive at the same group-to-RP mapping. This mapping is then broadcast to all devices listening to the 224.0.1.40 group.




You can configure a set of routers to be the candidate RPs as follows:

 ip pim [vrf vrf-name] send-rp-announce {interface-type interface-number | ip-address} scope ttl-value [group-list access-list] [interval seconds] [bidir]

If there are multiple RPs advertising  the same information, the RP mapping agents elects the candidate RP with the highest IP address as the RP.