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Cisco 10000 Series Router Quality of Service Configuration Guide
OL-7433-09
Chapter 15 Oversubscribing Physical and Virtual Links
Interface Oversubscription
Interface Oversubscription
Interface oversubscription enables service providers to assign a total committed information rate (CIR)
to a given port that is greater than the speed of the port. In this way, instead of supporting only
unconditional reservation of network bandwidth to VCs, the Cisco 10000 series router can statistically
guarantee bandwidth to the VCs, thus improving network utilization.
While interface oversubscription offers distinct advantages, it has the potential of degrading system
performance during congestion. The packet scheduler directs traffic to a physical port. Without
oversubscription, the packet scheduler reserves scheduling resources for each VC so that the total
amount is less than or equal to the capacity of the network. With oversubscription, the scheduler assigns
its resources at a rate higher than the transmission capacity of the network. This mismatch of rates
between the scheduler and the network results in reduced scheduling efficiency such as the following:
•
The scheduling resources that are in excess of the physical capacity fail to perform productively
because of the network’s refusal to accept new packets during congestion.
•
Congestion at the physical layer causes the scheduler to retry packet transmissions until accepted by
the network, which further reduces the scheduler’s productivity. Prolonged congestion also
interferes with the scheduler’s operations to serve each traffic class according to its bandwidth
commitment.
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. Without oversubscription, you loose the benefits of
oversubscription such as the following:
•
Lower cost for port connection
•
Network bandwidth is available if and when you need it
•
The statistical multiplexing that is used with oversubscription saves money because data traffic
tends to be bursty
•
The statistical multiplexer assigns bandwidth on a circuit as needed, thus saving bandwidth
Feature History for Interface Oversubscription
Cisco IOS Release
Description
Required PRE
Release 12.0(25)SX
The interface oversubscription feature was introduced on
the router.
PRE1
Release 12.3(7)XI
This feature was introduced on the PRE2.
PRE2
Release 12.2(28)SB
This feature was integrated in Cisco IOS
Release 12.2(28)SB for the PRE2.
PRE2
Release 12.2(31)SB2
This feature was introduced on the PRE3 and uses
statistical multiplexing on ATM, Frame Relay, and IEEE
802.1Q networks.
PRE3
