TGW Route Table entries

Question

I just saw a [session][1] about Advanced Architecture with TGW where we show an example of a TGW with 4 attachments (2 VPCs, 1 DGW, 1 VPN). All attachments are associated with the single TGW route table and propagate their routes to it. Nevertheless the RT only shows 3 entries. The presenter says that’s due to DX is leveraged as primary connection to onPrem here and VPN only as a backup.
In my understanding of path selection behavior DX propagated routes are preferred over VPN propagated routes. So what is the reason that the RT of the TGW does not show all 4 entries?  
I’ve built that setup in a lab environment and can confirm that you only see 3 entries there - VPN is missing. Only when you failover to VPN, the DX propagated routes are not shown anymore in the TGW RT.

[1]: https://youtu.be/S9fEydjJ9qo?t=844

Accepted Answer

This is a "normal" behavior that you can also see with physical router. If multiple routes to the same destination are received, they only place the one with the highest priority in their routing table.

On a high level this is due to the decoupling of routing information base (rib) and forwarding information base (fib). The rib takes care of the path selection, e.g. calculates the best path for each known destination (based on different rules and algorithms) and places only this one in the fib, so if there is a packet received that needs to be forwarded the router only needs to look up the next hop in the fib and forwards the packet without the need to calculate the best path every time.

I guess it's just the same for TGW route table, I hope someone here can confirm that :-)

TGW Route Table entries

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