DELL N3000 User Manual

Configuring Data Center Bridging Features

The minimum bandwidth setting can be used to override the strict priority 

and weighted settings. The highest numbered strict priority queue will 

receive no more bandwidth than 100 percent minus the sum of the minimum 

bandwidth percentages assigned to the other queues. If used, it is 

recommended that minimum bandwidth percentages only be set high 

enough to ensure a minimum level of service for any queue; i.e., the sum of 

the minimum bandwidth percentages is a fraction of 100%. This ensures that 

the system can respond to bursts in traffic. Setting the minimum bandwidths 

such that they sum to 100% effectively sets the scheduler such that no queue 

can exceed its minimum bandwidth percentage when congestion occurs.

Each CoS queue in the first level scheduler is mapped to one of the three 

traffic class groups in the second level scheduler. There, frames are serviced 

using the TCG configuration. The minimum bandwidth guarantee is first 

calculated across the TCGs. Strict priority TCGs are scheduled first but have 

their bandwidth reduced by the minimum bandwidth guarantees configured 

on other TCGs. Strict priority TCGs are scheduled from highest numbered 

TCG to lowest. When all TCGs have met their minimum bandwidth limits 

(or the queues are empty), TCGs that have not met their maximum 

bandwidth limit are scheduled. Once the limits for a TCG are satisfied 

(maximum bandwidth, no frames available for transmission, etc.), the 

scheduler moves to the next TCG.
If no minimum or maximum bandwidth limits are configured, TCGs are 

serviced by the second-level scheduler using the configured TCG weights to 

define the relative bandwidth allocation among the TCGs. When an egress 

port is congested, packets are selected for discard using the configured tail-

drop or WRED discipline. Minimum TCG bandwidth, maximum TCG 

bandwidth, and TCG weights are metered to within approximately 3% of the 

link bandwidth.
In the case that all TCGs are configured as strict priority, inter-TCG 

scheduling reverts to Weighted Deficit Round Robin scheduling based on the 

configured TCG weights.

CoS queue 7 is reserved for internal traffic. Non-strict priority CoS queues 

are serviced with WDRR scheduling using the bandwidth available after strict 

priority traffic is serviced.