Expert Clinic - Flattening Ethernet

With Andrew Buss, originally published on The Register

Published/updated: September 2011

The concept of an IT service used to be defined as an individual, often silo’d, application running on dedicated hardware. With static infrastructure, traffic was fairly predictable, moving between applications on the server and client. As a result, the Ethernet-based data network tended to be optimised around this type of traffic flow and so the network evolved into hierarchies resembling a tree.

This architecture is well suited to these predictable traffic patterns between dedicated server and client. However, performance tends to suffer when traffic has to move between trees in the network – as happens when servers in different network segments need to communicate with each other.

At the same time, as Ethernet networks greatly expanded in size, switching based on Layer 2 became difficult to scale as it was broadcast-based. Protocols such as Spanning Tree helped, but ultimately the management, reliability and traffic overhead led to limited impact. Techniques such as VLAN segmentation also assisted, but introduced even more complexity, such as the need to route traffic between VLANs. This resulted in the emergence of Layer 3 Ethernet switches that work natively with IP to enable routing between VLANs in hardware.

Meanwhile, storage was moving out of the server, from internal direct-attached disks to a networked pool of storage. This SAN (Storage Area Network) had to support many servers connecting to many storage assets. To be effective, any node on the storage network must be able to communicate effectively with any other node on the network.

This led to the implementation of a different type of network architecture to traditional Ethernet – a fabric network designed for the needs of storage systems, where the network has a consistent state that is optimised to support point-to-point communications between any two nodes on the network and which has single step lookups to forward traffic.

Over that last few years there has been a growing shift in how IT services are delivered. The old, static approaches are progressively giving way to a service-centric approach that is defined by the needs of the business. The service is delivered using a set of building blocks that can be assembled to provide the service efficiently and effectively. A large enabler of this has been the emergence of virtualisation to allow service instances to be spun up or down anywhere on the network as required.

As a result network traffic between client and server is becoming less predictable; perhaps more importantly critical inter-server communications is becoming more widely used while virtual machine traffic is exploding. On top of this storage traffic is ballooning and in a growing number of cases also moving onto Ethernet.

Ethernet networks - as traditionally implemented - have arguably become too complex to support dynamic services and as a result of this the underlying Ethernet network architecture needs to evolve to become more fabric-like in its characteristics and architecture, at least for certain areas of the data centre network where there is a lot of server and storage traffic.

A big step required in order to move to a fabric is to “flatten” the network and remove the “trees” that make point-to-point communications difficult. The ultimate goal would be to have a completely flat fabric with no tiers whose main purpose is to aggregate traffic and transport it between network segments. However, in most cases this will not be cost-effective to implement, or realistic for organisations to adopt widely in the medium term.

The goal will be to try and collapse parts of the network by removing one or more of the network tiers. In effect this would provide “islands” of fabric. This can be achieved by deploying some of the latest Ethernet fabric switches without having to have a “roots and branch” overhaul of the entire network.

Another change with the move to a flat fabric network is that extended support for Layer 2 networking is needed. As we’ve seen, Layer 3 switching helps in situations where IP is the dominant protocol. However, the rise of storage traffic on Ethernet as well as the growing prevalence of virtual machines means that non-IP traffic is getting to be a lot more significant and needs to be catered for at Layer 2 across different network domains. This requires new equipment with support for enhanced Layer 2 protocols such as FabricPath to remove the need for Spanning Tree on large, segmented network implementations.

Whichever direction your data centre network is headed, the long term trend in IT is towards a more dynamic and converged IT infrastructure. Having a long term view on supporting non-IP traffic effectively over Ethernet should be high up on the list in order not to be locked into an architecture that is difficult to reconfigure or scale.


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