Routing Application#

Datacenter networking (and any networking for that matter) is all about providing connectivity for clients. For the sake of this discussion, clients are computers. In reality, clients can be anything from mobile subscribers to firewalls, mainframes, badge readers, barcode scanners, security cameras, and so much more.

In its simplest form, two computers can talk to each other by connecting them with a cable:

This is technically a network with two hosts, but isn't very flexible: what if a third PC is connected? We could connect computer B to computer C, but that too has big limitations: if computer B shuts down, computer A can no longer communicate with computer C:

We need a device that provides point-to-multipoint connectivity: either by replicating every packet to all other clients (hub) or by intelligently passing the packets to the right computer (switch).

We're getting somewhere now! A collection of networking equipment and physical links that enable computers to talk to each other without restrictions is called a broadcast domain or bridged domain, and operates entirely on layer 2 traffic. As our network grows though, we will need to up our security and isolate different groups of clients.

For example, we may want to add a bunch of security cameras that we want to separate from our main computer network. That is where VLANs come into play: VLAN tags are numbers that are added to the ethernet header of a packet, and isolate traffic that is tagged with different VLANs.

But we have a problem: now that the two broadcast domains are isolated from each other, our computers can't connect to the cameras anymore. To fix this, we need a router that routes connectivity from one IP subnet to another. This is the first time that our networking equipment needs to become aware of the IP addresses that the hosts will use to communicate: previously, the network acted as a mailbox, whereas from now on the network behaves more like a postman.

The network has now grown so much that the clients can no longer all connect to the same network device. The network turns into a proper datacenter.

For reasons beyond the scope of this explanation, exchanging IP reachability information using BGP as a route exchange protocol in this way will not work. We need a protocol that allows broadcast domains to stretch over multiple networking switches.

In the datacenter world, EVPN is most frequently used for the exchange of service routes: it is a very flexible routing protocol that uses transport tunnels as a next hop instead of IP addresses. These transport tunnels are set up between two routers and support traffic from multiple different services, ensuring traffic remains isolated at the remote end of the tunnel.