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Cisco 10000 Series Router Quality of Service Configuration Guide
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Oversubscribing Physical and Virtual Links
Asynchronous Transfer Mode (ATM), Frame Relay, and IEEE 802.1Q Ethernet networks offer
connection-oriented, circuit-based data transfer. Such networks provide statistical multiplexing of traffic
from all the virtual circuits (VCs) sharing the physical port. Some VCs have a built-in method of traffic
capacity. Others like Frame Relay and Ethernet are just best effort. Optionally, a provider may choose
to reserve bandwidth for such a VC through configuration. Once a VC has a defined bandwidth, the
router strives to service the VC traffic at the specified transmission rate, and optionally provides
differentiated service to packets of that VC.
While provisioning bandwidth to virtual circuits, the provider has a choice of offering unconditional or
statistical guarantees. When the sum of the transmission capacities of VCs falls within the bandwidth of
the physical network, it never congests; therefore, each VC receives its bandwidth reservation regardless
of the traffic pattern of any other VC on that network. On the other hand, when the total assigned
bandwidth exceeds the physical capacity, the router allocates each VC bandwidth reservation as long as
a limited number of VCs activate at one time. The former offers unconditional service at the expense of
underutilization of the physical capacity; the latter takes advantage of statistical multiplexing to provide
better network utilization at the expense of degraded service under congestion.
To improve network utilization of otherwise underutilized shared networks, the Cisco 10000 series
router offers oversubscription functionality. Oversubscription helps to offset infrastructure costs and is
an integral part of networking topologies.
This chapter describes interface and virtual circuit oversubscription and includes the following topics:
•
Interface Oversubscription, page 15-2
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ATM Virtual Circuit Oversubscription, page 15-3
•
Frame Relay PVC Oversubscription, page 15-7
•
Virtual LAN Oversubscription, page 15-9
•
Oversubscription and Hierarchical Policies, page 15-10
•
Configuring Oversubscription, page 15-10
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How the Router Determines VC Weights, page 15-20
•
Configuring VC Weighting, page 15-28
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Configuration Examples, page 15-38
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Verifying Oversubscription and VC Weighting, page 15-41
•
Related Documentation, page 15-44
