This post will cover multicloud networking integration between multiple public clouds and on prem network. Imagine four clouds three being AWS, Azure and GCP and the fourth being the on prem private cloud which is basically a Data Center network.
All these four clouds will be glued together somehow and that glueing will be the multicloud scenario. The basic requirements would be to have switching, routing, firewalling and load balancing equipment present within the glueing network between the four clouds.
Switching would be present to trunk layer 2 between IP endpoints. Routing and routing protocols like BGP would be there to exchange the IP endpoints reachability information to populate routing tables and get the Nexthops.
IP planning would be involved in the sense that the On Prem and the three public clouds dont have duplicate conflicting IP address spaces and there aren’t two endpoints in the network which are generating packets with the same source IP address.
In essence if there a single routing table present in your environement which has routes for all three Public cloud endpoint subnets and also the routes for the on prem DC network then you have multicloud established.
Wherever this routing table exists from that location there will be Layer 2 swithcing links and trunks into the three clouds and On-Prem until the trunks reach the other routing tables within the clouds, be it Azure VNET routing table, AWS/GCP VPC routing tables or On-Prem DC Routing Tables.
This multi-cloud environment is somewhat similar to large Service Provider public internet networks we are all familiar with where each large SP can be considered a cloud in itself with routes being exchange with the other large SP i.e. similar to cloud routes over BGP.
The SP environment are mostly used for traffic passing through whereas in the multi-cloud enterprise environemnt there are Data Sources and Data Sinks in either the On-Prem or in the Public Clouds. There is also the difference that the glueing network in the middle will have firewalling too.
Lets say there is a new connection required to a VPC subnet in a AWS region. Firstly the layer 2 would be provisioned over the AWS Direct Connect either directly with AWS or with partners like Megaport. For the majority of the cases the on-prem device which connects to the direct connect service will be provisioned with a new VLAN.
Once this is done this layer 2 will be trunked to the on prem device where IP endpoint is provisioned and the routing table exists. This could be a firewall or a router. This is where the packets will decide on the next hops.
On-Prem firewall filtering is in the path where the different DMZ regions, different IP Subnets and L4 Ports are allowed or disallowed to communicate with each other. If the On-Prem device with the routing table containing the multi cloud routes is a firewall things are simpler in the sense that the firewall filters are present on the same device and the different clouds are treated as different DMZ zones.
This multicloud networking scenario is a routing environment which has multiple routing domains as spokes linked via a hub site. This hub site is the on-prem glueing routing table. There would be the addition of firewalling capability within this environment so as to be able to govern and allow/disallow traffic between these environments. Another addition could be a load balancer within the glueing on-prem environment.
This load balancer would spray traffic onto either on-prem DC subnets IP endpoint servers or onto the public cloud subnets housing cloud servers. This would mean that there will be public facing IPs which receive the traffic which is natted onto Private IPs and then it is loadbalanced onto the multiple server endpoints be it in Public clouds or in On-Prem DC.
So the load balancer would have the load balanced front end IP to Server IP bindings going towards either a public cloud endpoint or an on-prem endpoint. This would mean that the load balancer connects to the glueing routing table entity as well to send/receive traffic to server IPs.
This mix of route, switch, firewall, load balancer is an example of a typical multicloud network connecting multiple public clouds.