Transit Gateway - number of prefixes from TGW-CGW

Question

With reference to the Direct Connect limit "[Number of prefixes per AWS Transit Gateway from AWS to on-premise on a transit virtual interface][1]" (20). My interpretation of this is that if you had, say, 25 VPC attachments on a TGW, 5 CIDRs would not be advertised to the Customer Gateway.

Are the 5 prefixes just suppressed from BGP Updates? Does the BGP neighbor relationship take a hit (similar to exceeding the inbound prefix limit)?

How are others designing around this limitation in situations where the customer has > 20 VPC attachments?

[1]: https://docs.aws.amazon.com/directconnect/latest/UserGuide/limits.html

Accepted Answer

When a customer associates a Transit Gateway to Direct Connect Gateway, customer provides a list of prefixes which are advertised to on-prem and this is where limits are enforced. If customer needs to add or delete the routes advertised, they can use the "UpdateDirectConnectGatewayAssociation" API to change it.

My general advice to customers is to summarize as much as possible and use contiguous blocks for their cloud footprint holistically.

Transit Gateway - number of prefixes from TGW->CGW

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