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Chapter 9: Shared Shaping Overview
that includes a shared-shaping rate command cannot be associated with a group
node.
ATM and Shared Shaping
When you configure shared shaping with ATM, be sure to consider the following
behaviors.
Sharing Bandwidth with the SAR
On ATM line modules, providers can use the SAR to implement bandwidth sharing
for VCs. When the SAR is operating in default mode (that is, when the no
qos-mode-port command is in effect), the SAR backpressures the VC node in the
default traffic-class group, but traffic that is queued through a named traffic-class
group is unaffected by VC backpressure. In the absence of voice and video traffic,
the VC runs data traffic at the shared rate. When voice and video traffic start
streaming, the SAR backpressures just the VC node in the default traffic-class group,
thus sharing the bandwidth.
However, providers need to configure shared shaping on more than just ATM VCs.
The SAR cannot support shared shaping per virtual path on ATM, and there is no
SAR on Ethernet line modules. The shared shaper implemented in the HRR scheduler
can support shared shaping for all these different configurations.
Shared Shaping and Low-CDV Mode
JUNOSe releases before Release 6.0.0 implemented a carve-out scheduling model.
If you configured multiple scheduler nodes for a VC or VP, the router added together
the shaping rates for each scheduler node and shaped the corresponding VC or VP
tunnel in the SAR to the sum of the rates. This implementation forced a strict-priority
carve-out model for a logical interface, because the best-effort traffic cannot share
unused bandwidth from the strict-priority traffic-class group.
Beginning with JUNOSe Release 6.0.0, the router synchronizes the SAR rate for a VC
or VP to the shared-shaping rate for the best-effort scheduler node for the VC or VP,
so that the default behavior for low-CDV mode becomes shared shaping. Applying
shared shaping to the best-effort queue does not synchronize the rate for the
corresponding VC or VP in the SAR.
JUNOSe releases before Release 6.1.0 had a different behavior than the current shared
shaping model when multiple traffic-class groups were configured in low-CDV mode.
In those releases, the shaping rates of the VC nodes in each group were added
together, and the corresponding VC queue in the SAR was shaped to the sum. The
same algorithm was used for shaping VP tunnels in the SAR the shaping rates of
all VP nodes in the hierarchical scheduler were added together to shape the VP tunnel
in the SAR. This behavior implements a carve-out model for scheduling into VPs and
VCs and generally is not as desirable as the shared shaping model supported in
JUNOSe Release 6.1.0 and later releases.
Beginning with JUNOSe Release 6.1.0, low-CDV mode causes SAR shaping of VCs
and VPs only when you specify the shared-shaping-rate command for the best-effort
VC or VP node in the HRR scheduler.
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Guidelines for Configuring Simple and Compound Shared Shaping
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Troubleshooting
