Situation 

• Account A owns the VPC, and Account B owns EC2 instances. You want to connect Account B’s EC2 instances to the VPC owned by Account A, but routing table changes are not possible. 

  • Account A cannot view Account B’s EC2 instances. 

  • Account B cannot change Account A’s VPC settings. 

Solution 

• Instead of using instance-id, use eni-id. 

• Reference 

  • SAP on AWS High Availability with Overlay IP Address Routing
    https://docs.aws.amazon.com/sap/latest/sap-hana/sap-oip-configuration-steps-for-aws-transit-gateway.html#sap-oip-step-2.-configure-routing-for-aws-and-corporate-networks