Effects of virtualization and cloud computing on data center networks

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file block sizes. If your network is oversubscribed, your ability to do live migrations might also be
compromised because of the large bandwidth capabilities required.
Oversubscription can also lead to requirements for controlling traffic with QoS mechanisms such as
enhanced transmission selection, minimum bandwidth guarantees, and maximum rate limits. Most of
these mechanisms are manual processes and require a substantial amount of management. Using
QoS schemes to manage a scarce bandwidth resource increases complexity in your network and
expands the associated management overhead.
Port Extension technology
IEEE is developing port extension technology as part of the draft IEEE P802.1BR, Bridge Port
Extension standard. It introduces a device called a “port extender” that is essentially a physical switch
with limited functionality, managed as a line card of the upstream physical switch. Products such as
the Cisco Nexus Fabric Extenders (FEX) and Cisco UCS FEX are examples of port extenders.
Port extension technology extends the difficulties of the existing hierarchical network by adding yet
another layer, forcing packets to go across multiple “hops” on the network. For example, Cisco
recommends that you configure the Fabric interconnect in “End Host Mode” in its UCS system. Using
this mode, VM-to-VM traffic in a UCS blade enclosure must travel from the NIC-A to FEX A to Fabric
Interconnect A up to an upstream switch back to Fabric Interconnect B, to FEX B and finally back to
NIC B (Figure 4). If the architecture is already oversubscribed, it adds even more congestion to the
network and aggravates the oversubscription problem.
Figure 4: Port extension technology adds an extra “hop” to the typical three-tier architecture and can magnify
congestion problems.
As data centers support more clustered, virtualized, and cloud-based applications requiring high
performance across hundreds or thousands of physical and virtual servers, port extension technology
just seems to add cost and complexity.