UDP Multicast and Let Cluster Choose the Multicast Address

This is the default setting. With this option selected, the Red Hat High Availability Add-On software creates a multicast address based on the cluster ID. It generates the lower 16 bits of the address and appends them to the upper portion of the address according to whether the IP protocol is IPv4 or IPv6:

UDP Multicast and Specify the Multicast Address Manually

If you need to use a specific multicast address, select this option enter a multicast address into the Multicast Address text box.

If you do specify a multicast address, you should use the 239.192.x.x series (or FF15:: for IPv6) that cman uses. Otherwise, using a multicast address outside that range may cause unpredictable results. For example, using 224.0.0.x (which is "All hosts on the network") may not be routed correctly, or even routed at all by some hardware.

Unicast

As of the Red Hat Enterprise Linux 6.2 release, Red Hat High-Availability Add-On nodes can communicate each other using the UDP Unicast transport mechanism. It is recommended, however, that you use IP multicasting for the cluster network. UDP unicast is an alternative that can be used when IP multicasting is not available. For GFS2 deployments using UDP unicast is not recommended.