DELL N3000 User Manual

Configuring Data Center Bridging Features

At time t2, a burst of LAN traffic is incoming at the rate of 4 Gbps, this burst 

is allowed to borrow the unused 0.5 Gbps bandwidth from SAN TCG and 

transmitted since the offered load of SAN is only 3 Gbps.
At time t3, when the offered load of IPC falls to 2 Gbps and the bursty LAN 

traffic is at 6 Gbps, the available bandwidth for SAN and LAN is 4 Gbps each 

according to the TCG weights, which are set as 50% each. However, ETS 

allows the LAN traffic to borrow unused bandwidth from SAN traffic, and 

then 5 Gbps of LAN traffic is transmitted.

Lossless traffic must map user priorities one-to-one onto CoS queues. 

Lossless traffic must always be carried within a VLAN-tagged frame to 

differentiate it from untagged best-effort LAN control traffic, such as ARP 

and LLDP.
Strict-priority CoS queues may starve other CoS queues of traffic if the 

offered load of the strict-priority CoS queues equals or exceeds the available 

capacity of the egress interfaces. It is recommended that either a CoS-queue-

level min-bandwidth setting be utilized to ensure a minimum amount of 

bandwidth is processed on the non-strict-priority queues if there is a 

possibility that the strict priority traffic is not limited in bandwidth by some 

other means.
It is recommended that the sum of the minimum bandwidth percentages 

allocated to the group of CoS queues mapped to a single TCG be less than or 

equal to the weight percentage, so that packets are not dropped due to the 

congestion in the TCG. 
The maximum bandwidth for a TCG should always be configured to be 

greater than the minimum bandwidth or the weight of the TCG. 
In the case where all TCGs are configured as strict priority, inter-TCG 

scheduling is Weighted Deficit Round Robin, using the configured weights.
Configuring CoS queues with a total maximum bandwidth that is less than 

the corresponding TCG weight will result in bandwidth being wasted in the 

secondary scheduler.
Configuring CoS queues with a total minimum bandwidth that is greater 

than the corresponding TCG weight will flood the secondary scheduler and 

result in the minimum bandwidth parameter effectively overriding the TCG 

weight setting.