Users Guide

Each PG can use the shared headroom pool only up to its PG headroom limit. The shared headroom feature provides the capability to share
the headroom buer between all the ingress ports or PGs. It also provides ways to learn statistical data on shared buer usage, thereby,
reducing the overall headroom buer allocation.
The PFC Shared Headroom feature provides the following two capabilities:
• Headroom Pool Management – Provides the capability to use the shared headroom buer between all the ingress ports or PGs to
reduce the overall headroom buer allocation.
• Headroom Pool Monitoring – Provides a mechanism to monitor the peak headroom buer consumed over a period of time, which in
turn helps in conguring a proper value for the shared headroom buer.
This feature also provides a mechanism to monitor the peak headroom buer consumed over a period of time, which in turn helps you to
congure a proper value for shared headroom buer.
Example Scenario
Consider a scenario where you want to congure two lossless queues on 12 40 Gigabit ports.
The following table illustrates the buer usage statistics when shared headroom is not used and each queue is allocated with a xed
headroom buer space:
Table 17. Buer usage statistics when shared headroom is not used
Parameter Description
PFC Shared Buer Size 208 KB
Buer-size parameter of dcb-buer-threshold per
lossless queue
94KB
Pause-threshold parameter of dcb-buer-threshold
per lossless queue
18KB
The Headroom value reserved per lossless queue 94 – 18 = 76KB
NOTE: 76KB is the headroom space that is required per PG [or a lossless
queue] on a 40 Gigabit port in a worst case scenario to guarantee
lossless behavior.
Total buer required for 2 lossless queues on 12
ports
12*2*94 + 208 = 2464KB
NOTE: Out of the 2464KB, 1824KB [12*2*76KB] is reserved only for the
headroom, which might never be utilized in most of the cases. An ideal
approach is to increase the PFC shared buer and reduce the buer
reserved for headroom. However, this approach subsequently reduces
the time to trigger PFC, thereby, reducing the eectiveness of PFC.
In the shared headroom feature, the main assumption is that not every PG uses the headroom buer at the same time. This approach
enables the system to save the headroom buer space that is reserved for every PG to guarantee lossless delivery during trac bursts. For
each PG, you can assign a lower value for headroom buer. This headroom buer is sucient enough to guarantee lossless behavior as this
buer is global and is shared among all the lossless queues.
The following table depicts the buer usage statistics when shared headroom is used:
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Data Center Bridging (DCB)