How does ACS 5.1 handle failover? Do you need two ACS devices?
In terms of configuration on the network device it is the same.Note that in ACS 5.1 configuration changes may only be performed on the primary server and get replicated to all secondary servers. If the primary fails the secondary can continue operating. However, in order to perform further configuration operations you need to promote the secondary to take the primary role and then continue performing configuration operations