Advertised routes
The on-premises side advertises prefixes saying "this destination is here" to Azure. Azure likewise advertises VNet prefixes to the peer.
Landing Zone Network Guide
5 Connecting On-premises and Azure / BGP
BGP is a mechanism where routers automatically exchange routes. Many routes appearing in Azure's effective routes are learned from ExpressRoute or the on-premises side.
Key points of this chapter
- BGP advertises reachable destinations
- Shown as Gateway Routes in Azure
- Without a return path, traffic becomes one-way
Explanation note
First explain the flow with a diagram, then supplement only the key points in text. For those unfamiliar with networking, showing where traffic flows before terminology is effective.
Do not register everything manually
Rather than manually entering all effective routes into routers, exchange necessary routes via BGP and filter out unnecessary ones.