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Functional Descriptions

VMDS-10488 VSC7511 Datasheet Revision 4.2

130

3.12.1

Scheduler Element

The following illustration shows the scheduler element.

Figure 41 •

Scheduler Element

By default, the scheduler operates in strict priority between all 12 inputs. The inputs are searched in the

following prioritized order: Input 11 has highest priority followed by 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, and 0.

In addition, the scheduler can operate either in a byte-based deficit weighted round robin (DWRR) mode

or a frame-based round robin (RR) mode. This is an overall configuration for the scheduler element and

cannot be selected per input. In DWRR mode, the participating inputs are given a weight and the

scheduler selects frames from these inputs according to the weights. The DWRR is byte-based and

takes the lengths of the frames into account. In RR mode, the participating inputs are selected one after

another. The RR is frame-based and does not take the length of the frames into account.

The scheduler supports a mixed mode where some inputs operate in strict priority and others operate in

either DWRR or RR. Any number of inputs can be assigned to either group but strict priority inputs must

always be selected from highest numbered inputs.

Each scheduler element has an associated leaky-bucket shaper at the output. The shaper limits the

overall transmission bandwidth from the scheduler element. Frames are only scheduled if the shaper is

open.

Each scheduling element determines whether it has a frame ready for scheduling based on status

information of the scheduling element’s inputs. For each input, the scheduling element knows if the input

has a frame ready for transmission and if the frame is ready due to a work conservation mode. The

overall scheduling algorithm within a scheduling element is as follows:

If the output shaper is closed, frames are only scheduled from the element if the element is enabled

for work conservation. Otherwise, frames are held back until the shaper opens.

If the output shaper is open or in work conservation mode, the scheduling element schedules

between inputs that are not work conserving by following the rules for the scheduler configuration:

strict inputs are scheduled first then round robin based inputs are scheduled according to either the

DWRR algorithm or the RR algorithm.

If no frames are scheduled during step 2, a second round of scheduling is performed. Inputs that are

work conserving become candidates for the second round of scheduling.

The hierarchy of scheduling elements is traversed from the element connected to the egress port through

to the element that connects to an egress queue by recursively deciding which input should be

scheduled.

156

80 through 87

11 (CPU)

157

88 through 95

Table 102 •

Scheduler Elements Numbering (continued)

Egress Port

Scheduler Elements - Level 1

Scheduler Elements - Level 2

Byte-based

DWRR

Frame-based

N-Strict

Output

Shaper

Inputs

Scheduler

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