Users Guide

Table 21. Buffer usage statistics when shared headroom is not used (continued)
Parameter Description
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 buffer 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 buffer and reduce the
buffer reserved for headroom. However, this approach subsequently
reduces the time to trigger PFC, thereby, reducing the effectiveness of
PFC.
In the shared headroom feature, the main assumption is that not every PG uses the headroom buffer at the same time. This
approach enables the system to save the headroom buffer space that is reserved for every PG to guarantee lossless delivery
during traffic bursts. For each PG, you can assign a lower value for headroom buffer. This headroom buffer is sufficient enough
to guarantee lossless behavior as this buffer is global and is shared among all the lossless queues.
The following table depicts the buffer usage statistics when shared headroom is used:
Table 22. Buffer usage statistics when shared headroom is used
Parameter Description
PFC Shared buffer size 208 KB
Buffer-size parameter of dcb-buffer-threshold
per lossless queue
18KB
Pause-threshold parameter of dcb-buffer-
threshold per lossless queue
18KB
Average Headroom value reserved per lossless
queue
35KB [Considering 76KB as worst case value]
Total Headroom buffer to be configured in
Shared Headroom Pool
12*2*35 = 840KB
Total buffer required for 2 lossless queues on 12
ports
12*2*18 + 208 + 840 = 1480KB
Shared PG headroom buffer approach saves 984KB from the traditional dedicated PG headroom approach for the same number
of lossless queues. The excess buffer can either be assigned to PFC shared buffer size (to reduce the PFC aggressiveness) or
to the lossy pool to effectively manage lossy traffic.
NOTE:
When only few lossless queues are configured, the Shared headroom pool must be configured carefully to avoid any
ingress MMU drops due to insufficient headroom buffer.
Configuring Shared Head Room Buffer
You can configure a shared head room pool limit, which is the threshold value for the shared head room pool size.
To configure the shared head room pool size:
Enter the following command in global configuration mode:
CONFIGURATION Mode
dcb pfc-headroom-buffer-size buffer-size
NOTE:
Earlier head room implementations are derived from the buffer-size and pause-threshold attributes of the dcb-
buffer-threshold profile. However, with the introduction of the shared head room feature, the buffer-size attribute is now
obsolete as the head room buffer is now global and is shared among all lossless queues or PGs. Any value configured for
buffer-size is internally replaced with value configured for the pause-threshold attribute. The peak-head room attribute
260 Data Center Bridging (DCB)