The underlay network for software defined access should be configured to ensure performance, scalability, and high availability.
Any problems that occur on the underlay network will affect the operation of the overlay network.
It is possible to use a Layer 2 network underlay design running spanning tree protocol, but it is not recommended.
The recommended design is a network underlay to use a Layer 3 routed design with IS-IS as the interior gateway protocol.
IS-IS offers operational advantages such as establishing neighbours without IP dependencies and peering using loopback addresses.
There are two modes that an underlay network can be ran in, manual underlay or automated underlay
This type of underlay network is configured and managed manually through a command line interface, rather than through Cisco DNA centre.
The advantage of the manual underlay is allows the network engineer to design the underlay around any special design requirements and can allow software defined access to run on legacy network technologies too.
The automated underlay is a fully automated network underlay. All aspects of the network are configured and managed by Cisco DNA centre’s LAN automation feature.
The LAN automation feature creates an IS-IS routed access campus design and uses the Cisco Network Plug and Play features to deploy unicast and multicast routing configurations for SD-Access.
The automation configuration removes the possibility of any engineer misconfigurations and reduces the complexity of the underlay network design.