How to: What are some of the AWS limitations?
There is a route table entry for every instance in the cluster which allows other nodes to route to the pods on that node. AWS has a soft limit of 50 routes per route table and a hard limit of 100 routes per route table. So with the default VPC networking you will run into one of these limits. Using one of the overlay networks (Flannel, Weave, Romana) can get around this limit. And kops-routing is a planned feature to do this directly from AWS nodes. A planned feature is for all these networking plugins to be drop in additions to an existing cluster.