Overlay Network

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An overlay network is network protocols that use encapsulation to provision virtual tunnels and networks without requiring reconfiguration of the underlying transport network.

• used to implement logical links between nodes or networks
• abstracts the complexity of the underlying physical topology
• e.g., VPN
• Other types of overlay network
  • use encapsulation protocols and SDN to create a logical tunnel between nodes or networks that might be located in different physical topologies
• allows for the segmentation of the same physical network
  • e.g., cloud provider can use an overlay network to isolate each tenant’s traffic from other tenants
• when used inside a datacenter,
  • implemented using virtual extensible LANs (VXLANs)

Virtual Extensible LANs

A virtual extensible LAN (VXLAN) uses layer 2 encapsulation to create an overlay network that runs on a layer 3 IP underlay network.

• Each overlay network is allocated a 24-bit VXLAN network identifier (VNI)
• A VXLAN Tunnel Endpoint (VTEP) server, switch, or router encapsulates the layer 2 frames tagged with the VNI in UDP packets
  • The UDP packets are routed over the IP network to another VTEP
  • receiving VTEP decapsulates the packets to extract and process the frames
• alternative to manually configuring connections between VNIs and VTEPs:
  • Ethernet VPN (EVPN) uses Border Gateway Protocol (BGP) to advertise VXLAN networks and nodes as routes
    • implements a control plane
      • for automated configuration and management of the overlay network

Datacenter Interconnect

• overlay network could span two geographically separate locations
  • e.g., connecting instances or logical networks hosted in two different datacenters

Data Center Interconnect (DCI) refers to ways of creating links between datacenters and hosts/networks in different datacenters.

• Datacenter services typically use cluster technologies and other applications that depend upon layer 2 adjacency
  • means that the clustered servers or VMs must be part of the same broadcast domain and subnet
    • even if they are in different datacenters
  • stretching layer 2 boundaries over physical fiber links between datacenters with hosts in the different locations configured as part of the same VLAN can generate complex broadcast and spanning tree issues
    • mitigated by using VXLAN and EVPN
      • to implement datacenter interconnects
      • EVPN allows servers to:
        • discover adjacent MAC addresses
        • forward data using an overlay network to tunnel traffic between them