Hello @Khushboo Kumari ,
Welcome to Microsoft Q&A Platform. Thank you for reaching out & hope you are doing well.
I understand that you would like to know what does the Layer 3 connectivity and Redundancy in Azure ExpressRoute mean.
Layer 3 connectivity:
Network layer is layer 3 of OSI Model. Layer 3 provides the network’s routing and switching technologies that create logical paths known as virtual circuits (VC), which are used for the transmission of data between network nodes. The main functions of Layer 3 include routing and forwarding.
Layer 3: https://osi-model.com/network-layer/
Microsoft uses BGP, an industry standard dynamic routing protocol, to exchange routes between your on-premises network, your instances in Azure, and Microsoft public addresses. We establish multiple BGP sessions with your network for different traffic profiles.
ExpressRoute is designed for high availability to provide carrier grade private network connectivity to Microsoft resources. In other words, there's no single point of failure in the ExpressRoute path within Microsoft network. There are 2 connections - Primary and Secondary for to maintain the redundancy of the ExpressRoute circuit throughout the end-to-end network.
Microsoft network is configured to operate the primary and secondary connections of ExpressRoute circuits in active-active mode.
To improve high availability, it's recommended to operate both the connections of an ExpressRoute circuit in active-active mode. If you let the connections operate in active-active mode, Microsoft network loads balance the traffic across the connections on per-flow basis.
Running the primary and secondary connections of an ExpressRoute circuit in active-passive mode face the risk of both the connections failing following a failure in the active path.
Kindly let us know if the above helps or you need further assistance on this issue.
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.