Extended networks require a layer 2 Virtual Private Network (L2VPN), which provides a secure communications tunnel between an on-premises network and one in your cloud SDDC.
Each end of this tunnel has an ID. When the tunnel ID matches on the cloud SDDC and the on-premises side of the tunnel, the two networks become part of the same broadcast domain. Extended networks use an on-premises gateway as the default gateway. Other network services such as DHCP and DNS are also provided on-premises.
You can change a logical network from routed to extended or from extended to routed. For example, you might configure a logical network as extended to allow migration of VMs from your on-premises data center to your cloud SDDC. When the migration is complete, you might then change the network to routed to allow the VMs to use VMware Cloud on AWS networking services.
Prerequisites
Verify that Layer 2 VPN tunnel is available. See Configure a Layer 2 VPN Tunnel in the SDDC.