A unicast flood can seriously affect the network performance. In addition to the regular network traffic, every NLB node sends out a heartbeat packets (each heartbeat packet contains about 1500 bytes of data). By default, a node sends a heartbeat packet each second and waits for five of those packets to be received until it considers the node as converged. In a unicast flood situation, any switches rebroadcast this heartbeat traffic to all switch ports just like the regular network traffic. For example, if your network has a 24-port or 48-port switch, and only two of those ports connect to NLB nodes, the switch may end up broadcasting significant network traffic to 22 (or 46) servers that don't need it.




How VMware And Microsoft Server Load-balancing Services work