Cluster Configuration

  All instances of l2tpns run the clustering code
  A cluster may consist of a single node
  Configuration (defaults in parenthesis):
     cluster_address (239.192.13.13)
     cluster_interface (eth0)
     cluster_mcast_ttl (1)
     cluster_hb_interval (0.5s)
     cluster_hb_timeout (15s)
     cluster_master_min_adv (1)
  Cluster nodes are homogeneous
  Two nodes for redundancy, seamless upgrade/restart: heartbeat compatability between minor versions
  More for load balancing
  Maximum typically restricted by router multi-path limit (6 + 1 for Cisco)