300-120 WLSI: Implementing Cisco Wireless Advanced Solutions

224.0.1.40 is the Cisco Auto-RP Discovery multicast address. After candidate RPs advertise their supported group ranges, an Auto-RP mapping agent selects the appropriate RP for each range and sends the resulting RP-to-group mappings to 224.0.1.40, allowing PIM routers to learn them automatically. 224.0.1.39 is the strongest distractor because it carries candidate-RP announcements to mapping agents, rather than the final RP-discovery mappings distributed to PIM routers.