last week I was at a customer and I had the chance to do some screenshots from two cluster which show how many heartbeat traffic a cluster can produce.
1. A two node cluster with four virtual machines. There you have a great heartbeat with 500 Kbit/s on the network. So pretty low.
2. A six node cluster with 215 virtual machines. Without any external action the cluster heartbeat has a continues traffic from around 60 MBit/s. I started some livemigrations and the heartbeat partly jumps up to 150 MBit/s.
So my conlusion, the heartbeat traffic multiplies with the load and number of nodes within the cluster.