[migrated] secondary CIDR blocks

[geoffcline]

Migrated from internal customer ticket

The notion that custom networking in EKS by adding secondary CIDR blocks to a VPC from the 198.19.0.0/16 ranges, in conjunction with the CNI Custom Networking feature is not the present case. According to VPC documentation, you cannot associate 198.19.0.0/16 range as secondary CIDR to any existing VPC CIDR range including CIDR from RFC 1918 range, CIDR from CG NAT or Publicly routable CIDR block (non-RFC 1918). See https://docs.aws.amazon.com/vpc/latest/userguide/vpc-cidr-blocks.html I also validated this to be true.

page: https://docs.aws.amazon.com/en_us/eks/latest/best-practices/custom-networking.html