Fencing is the ability of one node to immediately terminate a node that is in an indeterminate state. This is often referred to as STONITH ("Shoot The Other Node In The Head") and is used by a failing node to authoritatively shut down a node that is misbehaving.
The most common, and most compatible, option is 'Lanplus'.
It's important that no special characters (including a space character) are used in the username, password or Guest name. This is a known issue with the legacy IPMI code, and is not fixable on RHEL-6 based distributions.
If you have a policy that 'special characters must be used in all passwords', limiting them to (, ), ; and : does not cause any compatibility issues.