Multicast Routing - Part I Sparse Dense Mode

When multicast sparse dense routing is enabled then sparse mode is the preferred method of operation. If an RP is not configured or is unreachable for a particular multicast group then dense mode becomes the failover method.

The RP can be configured statically or via Auto-RP. Prior to release 12.2.7 PIM sparse dense mode was a pre-requisite for Auto-RP. This meant that dense mode failover was a feature even if not desired! To get around this from 12.2.7 onwards Auto-RP can be used with PIM sparse mode provided the 'ip pim autorp listener' global config command is used.