While almost every part of a modern datacenter can be considered mission-critical, the network is the absolute foundation of all communications. That's why it must be designed and built right the first time. After all, the best servers and storage in the world can't do anything without a solid network.
To that end, here are a variety of design points and best practices to help tighten up the bottom end.
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The term "network" applies to everything from LAN to SAN to WAN. All these variations require a network core, so let's start there.
The size of the organization will determine the size and capacity of the core. In most infrastructures, the datacenter core is constructed differently from the LAN core. If we take a hypothetical network that has to serve the needs of a few hundred or a thousand users in a single building, with a datacenter in the middle, it's not uncommon to find that there are big switches in the middle and aggregation switches at the edges.
Ideally, the core is composed of two modular switching platforms that carry data from the edge over gigabit fiber, located in the same room as the server and storage infrastructure. Two gigabit fiber links to a closet of, say, 100 switch ports is sufficient for most business purposes. In the event that it's not, you're likely better off bonding multiple 1Gbit links rather than upgrading to 10G for those closets. As 10G drops in price, this will change, but for now, it's far cheaper to bond several 1Gbit ports than to add 10G capability to both the core and the edge.
In the likely event that VoIP will be deployed, it may be beneficial to implement small modular switches at the edge as well, allowing PoE (Power over Ethernet) modules to be installed in the same switch as the non-PoE ports. Alternatively, deploying trunked PoE ports to each user is also a possibility. This allows a single port to be used for VoIP and desktop access tasks.
In the familiar hub-and-spoke model, the core connects to the edge aggregation switches with at least two links, either connecting to the server infrastructure with direct copper runs or through server aggregation switches in each rack. This decision must be determined site by site, due to the distance limitations of copper cabling.