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Introduction

While an AWS Transit Gateway (TGW) carries thousands of routes in the TGW route table, a TGW VPN has the same 100 route limit as the VGW VPN.

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Test Validation

In the following setup, we launch a Cisco CSR 1000v as the Customer Gateway and use it to attach a VPN connection to the TGW. The TGW and the CSR also run BGP between themselves. The detailed steps are here. tgw-route-limit After the VPN tunnel is created between TGW and CSR, run the following command at CSR to verify the BGP peer connection, where 169.254.11.233 is the TGW’s BGP ID: 
ip-10-10-0-106#show ip bgp neighbors 169.254.11.233 | include BGP state
  BGP state = Established
Turn on BGP debug on CSR and then configure CSR to advertise 101 routes to TGW through BGP. CSR shows the following BGP debug messages: 
*Dec 18 00:29:28.213: %BGP-3-NOTIFICATION: received from neighbor 169.254.11.233 6/1 (Maximum Number of Prefixes Reached) 7 bytes 00010100 000064
*Dec 18 00:29:28.213: BGP: ses global 169.254.11.233 (0x7F36A1AF6C60:1) Receive NOTIFICATION 6/1 (Maximum Number of Prefixes Reached) 7 bytes 00010100 000064
*Dec 18 00:29:28.213: %BGP-5-NBR_RESET: Neighbor 169.254.11.233 reset (BGP Notification received)
*Dec 18 00:29:28.213: BGP: ses global 169.254.11.233 (0x7F36A1AF6C60:1) Reset (BGP Notification received).
*Dec 18 00:29:28.213: BGP: 169.254.11.233 went from Established to Closing
The above messages show that when the TGW exceeds its route limit (100), it sends a BGP reset message to CSR. The BGP connection becomes idle after these messages. 
ip-10-10-0-106#show ip bgp neighbors 169.254.11.233 | include BGP state
  BGP state = Idle
This test indicates that TGW has the same BGP total prefix limitation (100) as a VGW.

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Summary

TGW VPN has a total 100 BGP prefix. In addition, a TGW cannot summarize Spoke VPC routes.