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Chapter 8 Configuring ATM Permanent Virtual Circuit Autoprovisioning
Variable Bit Rate Non-Real Time Oversubscription
Variable Bit Rate Non-Real Time Oversubscription
The Variable Bit Rate Non-Real Time (VBR-nrt) Oversubscription feature enables service providers to
improve network utilization of otherwise underutilized shared networks by leveraging statistical
multiplexing on ATM networks. Instead of supporting only unconditional reservation of network
bandwidth to VCs, the router offers VC oversubscription to statistically guarantee bandwidth to VCs.
The VBR-nrt Oversubscription feature assumes that congestion at the physical port never or rarely
occurs. For example, assume 10 VCs are configured to use 25 percent of the physical network
bandwidth. The full capacity of the network is reached if only four VCs attempt to transmit traffic. The
VBR-nrt Oversubscription feature is intended only for networks with low utilization in which congestion
is unlikely to exist.
In releases earlier than Cisco IOS Release Cisco IOS Release 12.3(7)XI1, a call admission check (CAC)
prevented you from assigning more bandwidth to virtual circuits (VCs) than a port’s total bandwidth.
The Cisco 10000 series router supported unconditional reservation of network bandwidth to VCs. When
the sum of the transmission capacities of VCs falls within the bandwidth of the physical network, the
network does not congest. Each VC receives its bandwidth reservation regardless of the traffic pattern
of any other VC on that network. However, VCs receive this unconditional service at the expense of
underutilization of the physical capacity of the network. Because each VC uses a fraction of the physical
capacity, unless a large number of VCs remain busy, the overall network utilization remains low.
In Cisco IOS Release Cisco IOS Release 12.3(7)XI1 or later, the VBR-nrt Oversubscription feature
enables you to specify the amount of oversubscription (oversubscription factor) you want to allow. The
CAC check is based on the oversubscription factor you specify and evaluated separately for both VCs
and VP tunnels into the port, and VCs into VP tunnels.
The oversubscription factor is also used to evaluate the amount of bandwidth allocated for unspecified
bit rate (UBR) VCs. Prior to Cisco IOS Release Cisco IOS Release 12.3(7)XI1, UBR VCs received the
bandwidth remaining after other VCs had been allocated bandwidth. The CAC check now adjusts the
bandwidth for UBR VCs based on the oversubscription factor. For example:
port speed- sum of the VBR VCs = aggregate UBR bandwidth
Note
You can apply a nested policy map to the main ATM interface to override this default equation and set a
specific bandwidth for the aggregate UBR queues.
Whenever the oversubscription factor is reduced, less bandwidth is available for VC creation. As a result,
a warning message appears indicating that some VCs might not be created. The VCs are not explicitly
removed from the configuration and remain up and functional until you reboot the router or reset the slot.
At this point, the VCs remain in the configuration but they are not up.
For optimal performance, configure the oversubscription factor as closely as possible to the sum of all
VCs. The system allows VCs to be added provided the total subscribed rate is less than or equal to the
port speed * over-subscription-factor.
CAC checks are disabled when the
no atm pxf queuing
command is configured on an interface.
For QoS-related information, see Chapter 13, “Oversubscribing Physical and Virtual Links” in the
Cisco 10000 Series Router Quality of Service Configuration Guide
.
