How Hierarchical Shaping Works - Cisco 10000 Series Configuration Manual

Traffic Shaping
Shaped UBRs on the ATM OC-12 Line Card
On an ATM OC-12 line card, when you configure UBR PVCs with a shaped value (UBR-PCR) and the
shaped value is greater than one-half of the line rate (for example, 299,520 Kbps), the following
limitations apply:
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How Hierarchical Shaping Works

The virtual time management system (VTMS) provides a single stage shaper at the packet level (IP plus
the appropriate MAC header) and the segmentation and reassembly (SAR) mechanism provides a single
stage shaper at the cell level. You can configure the SAR to shape and schedule at the VC level or VP
level. However, the SAR cannot shape and schedule at both the VC and VP levels simultaneously (for
example, shaping an individual VC and then shaping the aggregate VCs into a VP). When configured to
perform VP shaping, the SAR is configured with permanent virtual path (PVP) parameters.
Hierarchical shaping combines the single-stage VTMS and SAR mechanisms to provide a simultaneous
VC and VP shaping solution. When using hierarchical shaping, VC shaping occurs at the packet level
(using the VTMS) with the ATM VC functioning as a "link" scheduling mechanism.
From the perspective of the command line interface (CLI), you configure the shaped VC the same way
as you configure a regular VBR-nrt VC. Hierarchical shaping uses the CLI values you enter to configure
VTMS traffic shaping instead of SAR shaping. The VTMS schedules packets at the configured bit rate
for transmission to the appropriate line card SAR.
You configure the VP as a PVP. When you configure the VP as a constant bit rate (CBR) VP service,
hierarchical shaping uses the CLI values you enter to configure the SAR for ATM-based cell scheduling.
Hierarchical shaping shapes the ATM VC at the AAL5 packet layer on the parallel express forwarding
(PXF) processor using the VTMS. (Normally, the SAR provides this function at the cell layer in
non-PVP configurations.) The VTMS shapes the VC at the sustained cell rate (SCR) for VBR-nrt VCs
and at the peak cell rate (PCR) for CBR VCs. The VTMS uses the virtual path identifier (VPI) and the
virtual channel identifier (VCI) of VCs to associate shaped VCs with the appropriate PVP. The VTMS
places all VCs with the same VPI value in a common PVP and schedules the VCs to the SAR.
Cisco 10000 Series Router Quality of Service Configuration Guide
9-6
The number of VCs the OC-12 line card supports is up to one-half of the VC scaling limit of 16,384
VCs. Cisco IOS software counts each UBR-PVC above 299,520 as two VCs. Therefore, the active
VC count must be maintained at the following:
16,384 > (number of VCs at 299,520 and above * 2) + (number of VCs below 299,520)
At any time, if more VCs are active than the allowed number above, the SAR on the line card leaks
buffers, which results in a reduced buffer pool for active VCs and the SAR might fail if enough
buffers are lost. To recover the lost buffers, reboot the system.
The router allows you to enter shaping values between 299,520 and 599,040, which the SAR does
not support. The SAR performs shaping in the range of 599,040 and 299,520 to 299,538. If you
configure a shaping value between 299,528 and 399,032, the shape rate the SAR returns is unclear.
If you initially set a shaping rate of 599,040 and then change to another rate, or you initially
configure a shape rate and change to a rate of 599,040, the router accepts the command and the show
commands display the new rate. However, the SAR does not perform shaping correctly until the next
reload.
If you change the shaping rate from 599,040 to a lower rate, the LP shaper in VTMS allows the
average rate to be met. However, during traffic bursts, ATM-level shaping is not accurate.
Chapter 9 Shaping Traffic
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