Traffic shaping is a quality of service (QoS) mechanism that is used to manage the network traffic by shaping the traffic to a specified rate. Traffic shaping enables the network administrator to:
Control access to the available bandwidth.
Ensure that the traffic conforms to the policies.
Regulate the flow of traffic to avoid congestion that can occur when the transmitted traffic exceeds the access speed of its remote target interface.
Traffic shaping uses queues to constrain data bursts, limit peak data rate, and smooth jitters so that traffic will fit within the promised envelope. Traffic shaping limits the throughput by buffering excess traffic instead of dropping packets.
ATM VP/VC hierarchical shaping provides two levels of traffic shaping--per-VC and per-VP--to control or modify the flow of traffic on an interface. The shaping function also ensures that the traffic from one VC does not adversely impact another VC, thus preventing the loss of data. The traffic is shaped first at the VC level and then at the VP level.
If a service (or an
application) wants to use an ATM network to transport a particular kind of
traffic, it must first inform the network about what kind of traffic is to be
transported, and the performance requirements of that traffic. The ATM service
categories provide a method to relate traffic characteristics and QoS
requirements to network behavior. Hierarchical shaping requires that all VCs
belonging to a given VP have the same ATM service category. This hierarchy
ensures that packets are shaped first at the VC level and then at the VP level.
At the interface level, the priority is based on the ATM service category. The
service category is determined by the first PVC that is configured under a VP.
The priority is in the following order:
Constant Bit Rate
Unspecified Bit Rate + (UBR+)
Variable Bit Rate
Non Real-Time (VBR-nRT)
Variable Bit Rate (VBR-RT)
If no service
category is specified at the VC level, the default is unshaped UBR. The ATM
Hierarchical Shaping feature is supported for the all the above ATM traffic
For VBR-nRT traffic
the output sustainable cell rate (SCR) value that is configured will be taken
into account for VC shaping. For UBR traffic, the output peak cell rate (PCR)
value that is configured will be taken into account for VC shaping.
The ATM Hierarchical
Shaping feature supports over-subscription. Over-subscription occurs when the
sum of the configured rate of UBR and VBR PVCs exceeds the line rate.
No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature.
The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.