Note – According to Cisco Webinars, the underlay is already built in the CCIE Lab.


  • Overlay can run over any type of underlay.
    • Can be Layer 2 or Layer 3
      • Highly recommended layer 3.
        • ‘Lean and Mean’ underlay.
      • Spanning Tree is still needed if layer 2 is used as underlay.
  • Routing Protocol
    • Cisco recommends IS-IS.
    • Can be different routing protocol, commonly using different routing protocol if brown field.
    • SD-Access supports EIGRP, OSPF, and IS-IS.
    • Each edge device must advertise loopback interfaces into underlay.
      • Loopbacks are used to form VXLAN tunnels.
    • Shared Services
      • DHCP, DNS, Domain Services, DNA Center, WLC.
      • These services sit outside of fabric domain.
        • Underlay needs to be routable to shared services.
          • ie. to internal border node.
          • will not work unless internal and external border node are same device.
  • MTU
    • VXLAN requires an extra 50B for header.
      • 54B if there’s a VLAN tag.
    • Cisco Recommends MTU of 9100B for the entire underlay.
      • Can be devices not running as edge or border node
        • Middle ‘routing’ devices such as older switches just passing traffic.
  • Underlay link connectivity
    • P2P links between each switch in underlay.
    • Recommends 10Gbps of throughput between each switch.
    • Use BFD to improve failure detection.

None of this is necessary if Greenfield – use LAN automation with factory default IOS-XE switches. LAN Automation will build out IS-IS underlay.LAN Automation

LAN Automation:

  • Initial task is running discovery to import a Border node into inventory.
    • Once a border node is added to inventory, DNA can hop from the border node into neighboring devices to auto configure underlay.
    • Note – ‘ip routing’ needs to be configured on seed/border node before starting LAN automation.
      • Border is actually behind the scenes configuring itself as a DHCP server, handing out leases to other fabric devices, and then configuring them.
      • In addition the configurations are done with a PnP agent on the un-configured devices.
        • The additional fabric devices need to be completely factory reset.
        • Last button is ‘Stop Automation’.
          • Counter intuitive.

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