The basic peer-to-peer (both client and server function, or both mode) topology is the default configuration and has a single replica. However, you can also add additional replicas by configuring the replication domain.
This configuration represents the most consolidated topology, where the various system parts are collocated and requires the fewest server processes. When using this configuration, the most stable implementation is achieved when each node has equal capabilities (CPU, memory, and so on), and each handles the same amount of work.
It is also important to note that when using the peer-to-peer topology, replication must be possible within the replication domain for session data access and invalidation to occur properly. There must be 2 or more cluster members up at all times for a given replication domain. For example, if you have a cluster of 2 application servers, server1 and server2, that are both configured in the peer-to-peer mode and server2 fails. All of backup information for server1 is lost and replication is no longer possible.