Modifying The Vc Weight And The Vp Shaping Parameters; Example Scenarios Of Traffic Shaping Usage - Cisco 10000 Series Configuration Manual
Quality of service configuration guide
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Modifying the VC Weight and the VP Shaping Parameters
Modifying the VC Weight and the VP Shaping Parameters
In Cisco IOS Release 12.3(7)XI1, when you change the weight of a VC or the VP shaping parameters,
the SAR mechanism brings down the VC or VP and the session goes down.
In Cisco IOS Release 12.3(7)XI2 and later releases, you can change the VC weight, VP shaping
parameters, or VC shaping parameters without affecting the state of the VC or VP. Instead, the VC and
VP remain up and operational.
The dynamic parameters include the following:
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If VC members of a VP do not have a configured weight and you dynamically modify the VP rate, the
router dynamically adjusts the weight of the member VCs based on the associated tunnel's new rate. The
member VCs remain up as their weights are dynamically modified.
If you dynamically modify the tunnel VC's shaping parameters (for example, the PCR or SCR), the
router adjusts the weight of the tunnel VC based on the VC speed if no weight is configured for the VC.
If a VC weight is configured, the router adjusts the weight of the tunnel VC based on the configured
weight.
Example Scenarios of Traffic Shaping Usage
Traffic shaping is primarily used to control access to available bandwidth, to ensure that traffic conforms
to the policies established for it, and to regulate the flow of traffic to avoid congestion that can occur
when the transmitted traffic exceeds the access speed of its remote target interface.
The following describe scenarios in which you can apply traffic shaping to interfaces to regulate traffic:
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Cisco 10000 Series Router Quality of Service Configuration Guide
9-12
Overly aggressive burst sizes can lead to excess packet drops and throttle the overall output rate
–
(particularly with TCP-based flows).
ATM VP parameters such as peak cell rate (PCR) and cell delay variation tolerance (CDVT)
VC parameters such as weight, PCR, sustained cell rate (SCR), maximum burst size (MBS), and
CDVT
Controlling access to bandwidth—A policy stipulates that the rate of a given interface should not,
on the average, exceed a certain rate, even though the access rate exceeds the speed.
Differing access rates defined in a network—One end of a link in a Frame Relay network runs at
256 kbps and the other end of the link runs at 128 kbps. Sending packets at 256 kbps could cause
failure of the applications using the link.
Differing access rates defined on different DTEs—A link-layer network is giving indications of
congestion. The network has differing access rates defined on different DTEs attached to the
network. The network might be able to deliver more transit speed to a given DTE device at one time
than another. (This scenario warrants that the token bucket be derived and then its rate maintained.)
Offering subrate services—If you offer a subrate service, traffic shaping enables you to partition T1
or T3 links into smaller channels.
Preventing packet loss—Traffic shaping prevents packet loss. Using traffic shaping is especially
important in Frame Relay networks because the switch cannot determine which packets take
precedence. As a result, the switch cannot determine which packets to drop when congestion occurs.
It is critically important for real-time traffic that latency be bounded, thereby bounding the amount
of traffic and traffic loss in the data link network at any given time by keeping the data in the router
Chapter 9
Shaping Traffic
OL-7433-09
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