Server virtualisation results in a more complex network

Server virtualisation and blade servers may be driving data centre consolidation and changing the way networks are built; they tend to result in more complex network designs. While these technologies are delivering impressive efficiencies on the server side, their impact on rising numbers of network tiers in the data centre is significant.

However, by taking a holistic view of the network, newer architectures may be implemented that can simplify the network and make it more efficient.

Direct-attach architecture, utilising high-density modules in the aggregation switch, along with unique cabling solutions, eliminates multiple switching tiers, leading to reduced end-to-end latency, reduced oversubscription in the network, better power efficiencies and lower cost.

With server virtualisation, multiple server instances can be consolidated onto a single server in the form of virtual machines (VMs). The VMs on a single server communicate with each other through a virtual switch (vSwitch), software running in the virtualisation layer on the server that functions as a Layer 2 switch.

Blade server technology allows packing great computing power into a highly compact form factor. Each blade server can have multiple ethernet ports, each connecting into the backplane of the blade chassis enclosure and brought to the front panel using either a blade switch or a pass-through module. This module passes through each connection from every server out to the front panel for connectivity to the external network, often presenting a cabling challenge. However, the blade switch module simplifies things by switching traffic locally between servers within a blade chassis.

While increasing virtualisation is driving higher throughput demands down to the edge of the data centre, oversubscription due to the rising number of network tiers can lead to sub-optimal performance due to increasing choke points in the network. Each tier adds management overhead and troubleshooting complexity, since each switch has to be configured, monitored, maintained and updated with the latest software updates. Finally, each tier increases the network’s overall cost. Clearly a different approach to data centre network architecture is needed, and one approach follows.

Both virtual and blade switches have been driven from the server side of the data centre, although their significant impact on the network is often overlooked. While larger data centres traditionally have a 3-tier network ie, core, aggregation and access tiers, adding the blade and virtual switch tiers gives a 5-tier network. Each extra tier creates issues of increased end-to-end latency - typically each can add oversubscription, with ratios of 2:1 or 3:1 common.

Direct-attach architecture can connect blade servers directly into a very high-density aggregation switch, bypassing both the blade switch and the TOR or access switch. There are two main components:

1. Very high-density network aggregation switch modules provide 96 ethernet ports on a single I/O switch module. Since access and aggregation tiers are typically added to the network to increase fan-out, these high-density modules in a chassis form factor reduce the need for both an access and an aggregation tier in the network. The high density is supported by a high-capacity switch fabric and an overall switching capacity of just under 4 Tb.
2. MRJ21 technology consolidates six ethernet cables and connectors into one, significantly simplifying cabling.

The pass-through module can present cabling challenges, whose complexity MRJ21 cabling reduces significantly. In effect, the pass-through module in conjunction with MRJ21 cables can replace the blade switch, eliminating a tier of switching from the network.

MRJ21 technology also allows very high-density network switches to be built, resulting in very high-density fan-out. For example, a switch blade that uses 16 MRJ21 connectors on a single I/O blade can achieve a fan-out of 96 ethernet ports on a single blade. Up to eight of these blades can be stuffed into a single chassis providing connectivity for up to 784 GbE ports.

Using MRJ21 cables to connect ports from the pass-through module of the blade chassis directly into these high-density network switch modules can eliminate the TOR or access switch layer, since the high-density switch blades on the chassis provide the adequate fan-out required. In effect, the servers become directly attached to the aggregation switch tier, eliminating both the blade switch tier and the TOR or access switch tier.

Advantages of this architecture include: improved overall network latency by eliminating two active switching tiers from the network; significantly reduced oversubscription since both the TOR or access switch and the blade switch tiers added oversubscription; reduced power consumption from eliminating two switching tiers; reduced management complexity; and lower overall solution cost.

*Aaron Condon is the Acting ANZ Manager, Extreme Networks.