From the inception of data centers, there has been a need to standardize data transmission. At the current level of data transmission, we use something termed Ethernet VPN, or EVPN for short. EVPN, according to Juniper, is a network technology that utilizes a Layer 2 bridge in order to connect disparate networks together. When data centers were first developed, the topology was based on a couple of central links termed ‘God boxes’ that would be the point of entry and exit from the network into the wider world.
The main issue that occurred was that this type of network setup had problems with cross-communication, usually because of its reliance upon Layer 2 networking. As things progressed, data centers started to move away from Layer 2 and adopt Layer 3 networking instead, which relied on networking IP’s for routing. The problem here was that Layer 3 added complexity to the system that relied on simplicity to operate. But how could Layer 2 be used despite its massive shortcomings?
Exploring Layer 2 More Directly
Layer 2 was designed to be an easier to use system of routing. It requires very little overhead and companies usually charge more for Layer 3 routing since it utilizes packet-switching and other more complex methods of getting data from one computer to another. However, this simpler method of network traversal opens the door to a whole slew of other problems.
Whereas in more complex networks one is less likely to find a packet being lost in a loop, in a Layer 2 network, a simple oversight in not including a TTL mechanism can cause an entire network to meltdown thanks to a perpetual loop, where a packet is sent and received continually forever. As Cisco notes, there was no built-in prevention for Layer 2 loops, although Spanning Tree Protocol (STP) can be instituted as a fallback solution, discovering the possible routes that a packet can take and informing all packets of those routes.
The Drawbacks of Spanning Tree Protocol
While STP does provide a novel approach to the routing problem, it has an issue in the method in which it fails to deliver the packet. Normally, in a routing protocol, if a packet cannot be sent to a particular location then the protocol reports a failure and the packet isn’t sent. In STP, the opposite happens and communication is initiated from the recipient, meaning that in Layer 2 (where no inherent TTL solution exists) if an acknowledgement isn’t sent out fast enough because of a busy processor, then it could create a potentially unstable situation within the network, forcing a loop. Because of how Layer 2 Networks were designed, a single failure could doom the entire network.
Open Flow Models and Leaf-and-Spine
As technology developed, data centers started to adopt what was known as the Clos Topology. According to Packet Design, this Clos Network Topology is ideal for data centers since it introduces load balancing and scalability to the data center. In the current day, Clos topology has evolved into a leaf-and-spine design, making for a more efficient method of dealing with switching across the data center while introducing much-needed redundancy within the network. While these technologies made for a more efficient data center, the problem was that they weren’t backwards compatible and older applications which relied on the older network switching paradigm used in the best VPN services, were left out in the cold. And this is where Ethernet VPN makes its entry.
EVPN and Network Virtualization
Network virtualization is a term that refers to creating an overlay on top of a network topology. What it does, in a nutshell, is create a simple, end-to-end solution known as a tunnel, which is created by adding a new layer of headers to an existing packet. While the prevailing wisdom would have been to use a routing protocol, complexity was never desirable within the data center and so the decision was to use an IP-based technology, virtual extensible LAN (VXLAN). Combining VXLAN with Border Gateway protocol (BGP) which is already widely used on the internet, the data center could create the illusion of first-wave data center communication, which makes the data center’s network now fully backwards compatible.
EVPN Going Forward
EVPN alongside BGP provides a robust network for Layer 2 switching while at the same providing support for older systems that need technology that is no longer a part of the design of data centers. EVPN addresses a lot of the shortcomings that were introduced by Layer 2 switching and to this end it is invaluable to a growing data center. EVPN could provide very important insights to engineers seeking solutions that are both affordable and extensible for enterprise-wide network development.
