When we are talking about ExpressRoute, we are talking about a common Internet Service Provider (ISP) technology called Multi Protocol Label Switching (MPLS) or ISP IP VPN.
MPLS is a type of data-carrying technique for telecommunications networks that directs data from one network to the next based on short path labels rather than long network addresses. This technology avoids long and complex routing tables. The labels identify virtual links between distant nodes. MPLS can encapsulate packets of various network protocols; that's why it is named multiprotocol. MPLS supports nearly all common access technologies, including T1/E1, ATM, frame relay, and dark fiber connects, into points of presence or DSL.
The routing within those networks is based on Border Gateway Protocol (BGP) routing. BGP is a standardized gateway protocol designed to exchange routing and reachability information among autonomous systems on the Internet. The protocol is often classified as a...