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Network World - The emergence of 10 Gigabit Ethernet, virtualization and unified switching fabrics is ushering in a major shift in data center network design: three-tier switching architectures are being collapsed into two-tier ones.
Higher, non-blocking throughput from 10G Ethernet switches allows users to connect server racks and top-of-rack switches directly to the core network, obviating the need for an aggregation layer. Also, server virtualization is putting more application load on fewer servers due to the ability to decouple applications and operating systems from physical hardware.
More application load on less server hardware requires a higher-performance network.
Moreover, the migration to a unified fabric that converges storage protocols onto Ethernet also requires a very low latency, lossless architecture that lends itself to a two-tier approach. Storage traffic cannot tolerate the buffering and latency of extra switch hops through a three-tier architecture that includes a layer of aggregation switching, industry experts say.
All of this necessitates a new breed of high-performance, low-latency, non-blocking 10G Ethernet switches now hitting the market. And it won't be long before these 10G switches are upgraded to 40G and 100G Ethernet switches when those IEEE standards are ratified in mid-2010.
"Over the next few years, the old switching equipment needs to be replaced with faster and more flexible switches," says Robin Layland of Layland Consulting, an adviser to IT users and vendors. "This time, speed needs to be coupled with lower latency, abandoning spanning tree and support for the new storage protocols. Networking in the data center must evolve to a unified switching fabric."
A three-tier architecture of access, aggregation and core switches has been common in enterprise networks for the past decade or so. Desktops, printers, servers and LAN-attached devices are connected to access switches, which are then collected into aggregation switches to manage flows and building wiring.
Aggregation switches then connect to core routers/switches that provide routing, connectivity to wide-area network services,
segmentation and congestion management. Legacy three-tier architectures naturally have a large Cisco component – specifically,
the 10-year-old Catalyst 6500 switch – given the company's dominance in enterprise and data center switching.
Cisco says a three-tier approach is optimal for segmentation and scale. But the company also supports two-tier architectures should customers demand it.
"We are offering both," says Senior Product Manager Thomas Scheibe. "It boils down to what the customer tries to achieve in the network. Each tier adds another two hops, which adds latency; on the flipside it comes down to what domain size you want and how big of a switch fabric you have in your aggregation layer. If the customer wants to have 1,000 10G ports aggregated, you need a two-tier design big enough to do that. If you don't, you need another tier to do that."