Restrictions And Limitations For Atm Vc Oversubscription; Frame Relay Pvc Oversubscription - Cisco 10000 Series Configuration Manual
Quality of service configuration guide
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Chapter 15
Oversubscribing Physical and Virtual Links
To prevent oversubscription of the interface, enter the no atm oversubscribe command. For example,
the following configuration disables oversubscription of the ATM 4/0/0 interface. The previously
configured factor of 500 is configured on the interface, but the router does not allow the
oversubscription.
Router(config)# interface atm 4/0/0
Router(config-if)# no atm oversubscribe
Router(config-if)# end
To enable the oversubscription feature with the previously configured factor 500, enter the
atm oversubscribe command. For example:
Router(config)# interface atm 4/0/0
Router(config-if)# atm oversubscribe
Router(config-if)# end
Restrictions and Limitations for ATM VC Oversubscription
The following restrictions and limitations are applicable only if the port becomes congested:
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Frame Relay PVC Oversubscription
The Cisco 10000 series router supports the oversubscription of Frame Relay permanent virtual circuits
(PVCs). Oversubscription enables a service provider to assign a total committed information rate (CIR)
to a given PVC that is greater than the speed of the interface.
One of the goals of service providers is to save on expensive interface costs. The intermittent and bursty
nature of Frame Relay traffic practically guarantees that all of the Frame Relay PVCs are not going to
require the maximum committed information rate (CIR) at the same time. Therefore, Frame Relay traffic
is well-suited to oversubscribing available interfaces, thus reducing interface connection costs.
The router uses statistical multiplexing to assign bandwidth on PVCs as needed. This enables service
providers to provide service to a large number of customers over shared bandwidth, rather than dedicated
bandwidth and it enables customers to oversubscribe a PVC and still have guaranteed bandwidth through
the provider network even though the amount of guaranteed network bandwidth might exceed the
interface speed.
Network congestion is a concern when oversubscribing connections. If customers concurrently use all
of the network capacity, congestion can occur; however, the risk of congestion is low if you do not
oversubscribe too much and if you avoid oversubscribing networks that are likely to congest.
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To oversubscribe ATM VCs requires resources from the entire line card (for example, the 4-port
OC-3 ATM line card). If you configure oversubscription on only one port, the other ports can use
more resources than they were configured to use. As a result, some ports do not receive adequate
resources, which results in VC creation failures. Therefore, apply the atm over-subscription-factor
command on all ports of the ATM card.
Due to congestion on the physical interface, the accuracy of the bandwidth distribution between the
class queues degrades. For example, if you configure each of three queues at a distribution of 50,
30, and 20 percent, the actual distribution might be 45, 40, and 15 percent.
The distribution of bandwidth for each VC might be less than expected based on the speed of the
VC. Typically, low-speed VCs are allocated the expected bandwidth while high-speed VCs share the
remaining bandwidth equally.
The amount of bandwidth allocated for the priority queue or latency might be less than expected.
Cisco 10000 Series Router Quality of Service Configuration Guide
Frame Relay PVC Oversubscription
15-7
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