Multi-cloud networking is a single networking layer that joins a company's systems across several public clouds, such as AWS, Microsoft Azure and Google Cloud, and its own data centres, so that they can be connected, secured and watched as one network. It is usually sold as a subscription. Its buyers are large companies that use the cloud heavily.
Cloud computing means renting servers and storage from a provider instead of owning them. Many large companies rent from more than one provider. Each cloud has its own networking tools, with different names, settings and limits. A network engineer who knows AWS well may not know Azure. Connecting the clouds to each other, and to the company's own data centre, means building and maintaining separate links and rules in each one. A mix of public clouds and the company's own data centre is called hybrid cloud.
Take, for example, an online travel company in Chennai. Its booking website runs on AWS, in the Mumbai region. Its data analysis runs on Google Cloud. Its payment systems stay in its own data centre in Chennai, because the company wants full control over them. The booking website must reach the payment systems, and the analysis team must receive the day's bookings every night.
Without a multi-cloud networking layer, the network team writes separate routing tables and firewall rules in AWS, in Google Cloud and in the data centre. It keeps all three consistent by hand. When one is changed and the others are not, bookings fail.
With the layer, the team places a software gateway, a router that runs as a program, inside each cloud, and connects the data centre too. It then writes one policy in one console: "The booking website may reach the payment systems on port 443 only. The analysis account may receive booking data between 1 a.m. and 3 a.m." The platform turns this policy into each cloud's own settings and keeps them in step. All traffic between the clouds is encrypted. The console shows every connection on one map. So when payment requests slow down, the team can see which link is the cause.
Clouds and data centres are physically joined by cross-connects, short cables inside the same building to each cloud's network. Inside the company's own data centre, the traffic passes through data centre switches. Security teams can check what can reach what with attack path analysis, which builds a model of the whole network.
Multi-cloud networking is different from SD-WAN. SD-WAN joins offices and people to the network. Multi-cloud networking joins the clouds and data centres to each other.