Underlay Network and Overlay Network

What is Underlay Network and Overlay Network

 Today will discuss about the underlay and overlay networks and protocol used for these purposes. Most of the software defined networks are based on the underlay and overlay both in order to achieve the separate control and data planes in the network. Earlier we have the control and data plane within the same physical boxes but now separating data plane with the control plane is a huge shift of the design now a days.

Underlay network

Underlay networks or so-called Physical networks where traditional protocols are working.

Underlay Network is physical infrastructure above which overlay network is built. It is the underlying network responsible for delivery of packets across networks.

The underlay network is described by the physical devices and can be switches and routers in the network. All network components of the underlay must determine IP connectivity via the use of a routing protocol. The protocol used on the underlay devices are like OSPF, IS-IS, BGP for routing purposes.

For underlay network to setup a well-designed Layer 3 foundation inclusive of the campus edge switches to ensure performance, scalability, and high availability of the network.

Underlay Protocols: BGP, OSPF, IS-IS, EIGRP


Overlay network

An overlay network is formed on top of the underlay in direction to construct a virtualized network. The data plane traffic and control plane signalling is controlled within each virtualized network. Encapsulating user traffic in overlay networks using IP packets that are sourced.

Overlay networking is a method of using software to create layers of network abstraction that can be used to run multiple separate, discrete virtualized network layers on top of the physical network, often providing new applications or security benefits.

An overlay network is a virtual network which is routed on top of underlay network infrastructure, routing decision would take place with the help of software.

Overlay Protocols: VXLAN, NVGRE, GRE, OTV, OMP, mVPN


Summarization Between Overlay & Underlay Network

Description Underlay Network is physical infrastructure above which overlay network is built. An Overlay Network is a virtual network that is built on top of an underlying Network infrastructure/Network layer (the underlay).
Traffic Flow Transmits packets which traverse over network devices like switches and routers. Transmits packets only along the virtual links between the overlay nodes.
Deployment Time Less scalable and time consuming activity to setup new services and functions. Ability to rapidly and incrementally deploy new functions through edge-centric innovations.
Packet Control Hardware oriented. Software oriented.
Packet Encapsulation And Overhead Packet delivery and reliability occurs at layer-3 and layer-4. Needs to encapsulate packets across source and destination, hence incurs additional overhead.
Multipath Forwarding Less scalable options of multipath forwarding. In fact using multiple paths can have associated overhead and complexity. Support for multi-path forwarding within virtual networks.
Managing Multitenancy NAT or VRF based segregation required which may face challenge in big environments. Ability to manage overlapping IP addresses between multiple tenants.
Scalability Less Scalable due to technology limitation. Designed to provide more scalability than underlay network. E.g VLAN (Underlay Network) provides 4096 VLAN support while VXLAN (Overlay Network) provides up to 16 million identifiers.
Packet Delivery Responsible for delivery of packets. Offloaded from delivery of packets.
Protocols Underlay protocols include Ethernet Switching, VLAN, Routing etc. Overlay network protocols include Virtual Extensible LAN (VXLAN), Network Virtualization using Generic Encapsulation (NVGRE), Stateless Transport Tunning (SST), Generic Routing Encapsulation (GRE), IP multicast and Network Virtualization overlays 3 (NVO3).

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