How are the vnet subnets propagated to on-prem via express router ?

Hello @Gongya Yu ,

Welcome to Microsoft Q&A Platform. Thank you for reaching out & hope you are doing well.

I understand that you would like to know how the Vnet ranges are propagated to on-premises via Azure ExpressRoute.

As mentioned by @Marcin Policht below,

Routing exchange is over eBGP protocol. EBGP sessions are established between the MSEEs and your routers.

BGP utilizes a best path selection algorithm based on many factors including longest prefix match (LPM).

Refer: https://video2.skills-academy.com/en-us/azure/expressroute/expressroute-routing#dynamic-route-exchange

https://video2.skills-academy.com/en-us/azure/expressroute/expressroute-optimize-routing

For connectivity between virtual networks over ExpressRoute, you can refer the below doc:

https://video2.skills-academy.com/en-us/azure/expressroute/virtual-network-connectivity-guidance

Kindly let us know if it helps or you need further assistance on this issue.

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