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Sharing 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 65.

Table 65: Sharing 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 unit 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 66.

Table 66: First Example of Bandwidth Sharing

Logical Interface (Unit)	Calculation	Bandwidth
Unit 1	10 / (10+64+128+10) x 60 Mbps	2.83 Mbps
Unit 2	64 / (10+64+128+10) x 60 Mbps	18.11 Mbps
Unit 3	128 / (10+64+128+10) x 60 Mbps	36.22 Mbps
Unit 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 67.

Table 67: Second Example of Bandwidth Sharing

Logical Interface (Unit)	Calculation	Bandwidth
Unit 1	10 / (10+64+128+10) x 16.22 Mbps	1.93 Mbps
Unit 2	64 / (10+64+128+10) x 16.22 Mbps	12.36 Mbps
Unit 4	10 (10+64+128+10) x 16.22 Mbps	1.93 Mbps

Finally, Table 68 shows the resulting allocation of bandwidth among the logical interfaces when the port is configured with a 100-Mbps shaping rate.

Table 68: Final Example of Bandwidth Sharing

Logical Interface (Unit)	Calculation	Bandwidth
Unit 1	2.83 Mbps + 1.93 Mbps	4.76 Mbps
Unit 2	20 Mbps + 18.11 Mbps + 12.36 Mbps	50.47 Mbps
Unit 3	20 Mbps + 20 Mbps	40 Mbps
Unit 4	2.83 Mbps + 1.93 Mbps	4.76 Mbps

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