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# Shared Bandwidth Among Logical Interfaces

As a simple example showing how bandwidth is shared among the logical interfaces, assume that all traffic is sent on queue 0. Assume also that there is a 40-Mbps load on all of the logical interfaces. Configuration details are shown in Table 1.

Table 1: Example of Shared Bandwidth Among Logical Interfaces

Logical Interface (Unit)

Traffic Control Profile

WRR Percentages

Weights

Unit 1

PIR 100 Mbps

95, 0, 0, 5

10, 1, 1, 1

Unit 2

CIR 20 Mbps

25, 25, 25, 25

64, 64, 64, 64

Unit 3

PIR 40 Mbps, CIR 20 Mbps

50, 30, 15, 5

128, 76, 38, 13

Unit 4

Unshaped

95, 0, 0, 5

10, 1, 1, 1

When the port is shaped at 40 Mbps, because units 2 and 3 have a guaranteed rate (CIR) configured, both units 2 and 3 get 20 Mbps of shared bandwidth.

When the port is shaped at 100 Mbps, because units 2 and 3 have a guaranteed rate (CIR) configured, each of them can transmit 20 Mbps. On units 1, 2, 3, and 4, the 60 Mbps of excess bandwidth is shaped according to the values shown in Table 2.

Table 2: First Example of Bandwidth Sharing

Logical Interface (Unit)

Calculation

Bandwidth

1

10 / (10+64+128+10) x 60 Mbps

2.83 Mbps

2

64 / (10+64+128+10) x 60 Mbps

18.11 Mbps

3

128 / (10+64+128+10) x 60 Mbps

36.22 Mbps

4

10 (10+64+128+10) x 60 Mbps

2.83 Mbps

However, unit 3 only has 20 Mbps extra (PIR and CIR) configured. This means that the leftover bandwidth of 16.22 Mbps (36.22 Mbps – 20 Mbps) is shared among units 1, 2, and 4. This is shown in Table 3.

Table 3: Second Example of Bandwidth Sharing

Logical Interface (Unit)

Calculation

Bandwidth

1

10 / (10+64+128+10) x 16.22 Mbps

1.93 Mbps

2

64 / (10+64+128+10) x 16.22 Mbps

12.36 Mbps

4

10 (10+64+128+10) x 16.22 Mbps

1.93 Mbps

Finally, Table 4 shows the resulting allocation of bandwidth among the logical interfaces when the port is configured with a 100-Mbps shaping rate.

Table 4: Final Example of Bandwidth Sharing

Logical Interface (Unit)

Calculation

Bandwidth

1

2.83 Mbps + 1.93 Mbps

4.76 Mbps

2

20 Mbps + 18.11 Mbps + 12.36 Mbps

50.47 Mbps

3

20 Mbps + 20 Mbps

40 Mbps

4

2.83 Mbps + 1.93 Mbps

4.76 Mbps