A network engineer is designing hybrid connectivity using AWS Direct Connect and a Transit Gateway. A Transit Gateway is attached to a Direct Connect gateway and 19 VPCs in different accounts. Two new VPCs will be attached. The IP admin assigned 10.0.32.0/21 to the first new VPC and 10.0.40.0/21 to the second. The prefix list has room for only one more CIDR entry before reaching the quota. How should the engineer advertise AWS routes to on-premises while meeting these constraints?

The correct answer is to add 10.0.32.0/20 to the allowed prefix list. This is because 10.0.32.0/20 is a supernet that encompasses both 10.0.32.0/21 and 10.0.40.0/21. A /20 CIDR block contains 16 /24 blocks, and 10.0.32.0/20 covers the range from 10.0.32.0 to 10.0.47.255. Both 10.0.32.0/21 (10.0.32.0 - 10.0.39.255) and 10.0.40.0/21 (10.0.40.0 - 10.0.47.255) fall within this supernet. By advertising the single 10.0.32.0/20, the engineer uses only one entry in the prefix list, adhering to the quota constraint. The other options are incorrect because adding individual /21 CIDRs would exceed the quota, and "AWS managed prefix lists" are not the correct mechanism for advertising routes from a Transit Gateway to on-premises via Direct Connect; instead, an allowed prefix list associated with the Direct Connect gateway is used.