Restrictions And Limitations For Interface Oversubscription; Atm Virtual Circuit Oversubscription - Cisco 10000 Series Configuration Manual

Chapter 15 Oversubscribing Physical and Virtual Links

Restrictions and Limitations for Interface Oversubscription

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For more information, see the

ATM Virtual Circuit Oversubscription

The Cisco 10000 series router supports the oversubscription of variable bit rate-nonreal time (VBR-nrt)
and constant bit rate (CBR) virtual circuits (VCs), and virtual path (VP) tunnels. Oversubscription
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.
VC oversubscription 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. VBR-nrt oversubscription is
intended only for networks with low utilization in which congestion is unlikely to exist. If congestion
occurs on the PRE1 or PRE2, the router gives each VC an equal fair share of the bandwidth. If congestion
occurs on the PRE3, the router gives each VC a share of the bandwidth based on the configured weight
of the VC.
In releases prior to Cisco IOS Release Cisco IOS Release 12.2(16)BX, 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 12.2(16)BX or later, VBR-nrt oversubscription enables you to specify the amount
of oversubscription (oversubscription factor) you want to allow. Oversubscription of ATM interfaces is
disabled by default.
In releases prior to Cisco IOS Release 12.3(7)XI3, virtual path (VP) tunnel oversubscription does not
Note
behave as intended. Instead, oversubscription of VP tunnels (the number and bandwidth of VCs that can
be in a tunnel) is on by default and is not subject to any oversubscription factor. Oversubscription of VP
tunnels cannot be adjusted or turned off. However, in Cisco IOS Release 12.3(7)XI3 and later releases,
VP tunnel oversubscription is subject to call admission checks and is off by default. The CAC
mechanism checks all of the VCs and VP tunnels going to an interface, and all of the VCs going into a
VP tunnel. You can also specify the amount of oversubscription to allow.
The PRE3 allows the aggregate rate of ATM VCs to exceed the bandwidth of the interface or VP on
which the VCs were created based on the configured oversubscription factor on the ATM interface.
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Interface oversubscription can potentially degrade system performance during congestion due to a
mismatch of rates between the packet scheduler and the network.
With oversubscription, network congestion can occur if all of the network capacity is used
concurrently. However, the risk is low if you avoid oversubscribing a network that is likely to
congest and if you do not oversubscribe a service too much.
"Interface Oversubscription" section on page
Cisco 10000 Series Router Quality of Service Configuration Guide
ATM Virtual Circuit Oversubscription
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