Summarize Transit Gateway Prefixes across Multiple TGW

Question

Customer with 2 Transit Gateways, both associated with the same Direct Connect gateway.

They are looking to consolidate the prefixes on TGW A.

Currently they have:

TGW A
10.1.112.0/20, 10.1.144.0/20, 10.1.160.0/20, 10.1.176.0/20, 10.1.204.0/24, 10.1.224.0/20, 10.1.241.0/24 etc

TGW B
10.1.0.0/20, 10.1.16.0/20, 10.1.208.0/20

and would like to consolidate down to:

TGW A
10.1.0.0/16

TGW B
10.1.0.0/20, 10.1.16.0/20, 10.1.208.0/20

My question is, will prefixes be resolved using the longest prefix first? So routes matching the longer subnet masks on TGW B will still be routed, with everything else under the /16 mask being routed to TGW A?

AWS-User-0547701 · Accepted Answer

TGW is not the issue here explicitly. In Direct Connect Gateway you cannot currently have two Associations in DXGW with overlapping prefixes in their allow list

if you created TGW B Association first, the 3 routes (10.1.0.0/20, 10.1.16.0/20, 10.1.208.0/20) would be added to the allowed list in the Direct Connect Gateway association settings, when you attempt to place 10.1.0.0/16 into the Direct Connect Gateway TGW A Association, you will get an error:
"The requested allowed routes overlap with one or more existing allowed routes on the direct connect gateway"

Summarize Transit Gateway Prefixes across Multiple TGW

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