Generally, you may use VPN or Direct Connect to enable communications between on-premises data centers and VPCs at Layer 3. However, this may require network reconstruction, delay cloud migration, and cause service interruptions. For details, see Constraints on Layer 3 Networking Between an On-Premises Data Center and the Cloud.
Enterprise switches allow communications between on-premises data centers and VPCs at Layer 2, helping you dynamically and smoothly migrate workloads to the cloud. For details, see Advantages of Layer 2 Networking Between an On-Premises Data Center and the Cloud.
Constraints on Layer 3 Networking Between an On-Premises Data Center and the Cloud
You can use Direct Connect or VPN to enable communications between a VPC and your on-premises data center at Layer 3. In Figure 1, Direct Connect is used. If you want to use VPN, replace the Direct Connect connection with a VPN connection. Table 1 describes the pain points.
Figure 1 Layer 3 networking 
Table 1 Layer 3 networking description | Description | VPN or Direct Connect allows the communication between on-premises data centers and VPCs at Layer 3 through routes. |
| Pain Points | - The CIDR blocks of the on-premises data center and the VPC that are used for communication cannot overlap.
On-premises workloads communicate with each other using IP addresses instead of domain names. If the CIDR blocks of the on-premises data center and the VPC that are used for communication overlap, the on-premises network needs to be reconstructed before the cloud migration, which prolongs the cloud migration period, interrupts businesses, and increases O&M costs. - Workloads in a subnet have to be migrated together, and cloud and on-premises workloads in the same subnet cannot communicate with each other.
Dozens of different workloads are deployed on each subnet of the on-premises data center. If workloads are migrated by subnet, business continuity cannot be ensured. |
Advantages of Layer 2 Networking Between an On-Premises Data Center and the Cloud
To handle the pain points of cloud migration at Layer 3, you can use enterprise switches to allow the communication between on-premises data centers and VPCs at Layer 2. For details about the advantages of enterprise switches, see Table 2.
Figure 2 Layer 2 networking 
Table 2 Layer 2 networking description | Description | Enterprise switches establish a Layer 2 network between on-premises data centers and VPCs based on the Layer 3 network established by VPN or Direct Connect. |
| Advantages | - The CIDR blocks of the on-premises data center and the VPC that are used for communication can overlap.
An enterprise switch allows the network of the on-premises data center to remain unchanged even if the data center and the VPC have overlapping CIDR blocks. - Workloads can be migrated to the cloud on a server basis, and cloud and on-premises workloads in the same subnet can communicate with each other.
Workloads can be seamlessly migrated to the cloud to prevent any loss caused by cloud migration. |
