Each PG can use the shared headroom pool only up to its PG headroom limit. The shared headroom feature provides the capability to sharethe headroom buffer between all the ingress ports or PGs. It also provides ways to learn statistical data on shared buffer usage, thereby,reducing the overall headroom buffer allocation.The PFC Shared Headroom feature provides the following two capabilities:• Headroom Pool Management – Provides the capability to use the shared headroom buffer between all the ingress ports or PGs toreduce the overall headroom buffer allocation.• Headroom Pool Monitoring – Provides a mechanism to monitor the peak headroom buffer consumed over a period of time, which inturn helps in configuring a proper value for the shared headroom buffer.This feature also provides a mechanism to monitor the peak headroom buffer consumed over a period of time, which in turn helps you toconfigure a proper value for shared headroom buffer.Example ScenarioConsider a scenario where you want to configure two lossless queues on 12 40 Gigabit ports.The following table illustrates the buffer usage statistics when shared headroom is not used and each queue is allocated with a fixedheadroom buffer space:Table 17. Buffer usage statistics when shared headroom is not usedParameter DescriptionPFC Shared Buffer Size 208 KBBuffer-size parameter of dcb-buffer-threshold perlossless queue94KBPause-threshold parameter of dcb-buffer-thresholdper lossless queue18KBThe Headroom value reserved per lossless queue 94 – 18 = 76KBNOTE: 76KB is the headroom space that is required per PG [or a losslessqueue] on a 40 Gigabit port in a worst case scenario to guaranteelossless behavior.Total buffer required for 2 lossless queues on 12ports12*2*94 + 208 = 2464KBNOTE: Out of the 2464KB, 1824KB [12*2*76KB] is reserved only for theheadroom, which might never be utilized in most of the cases. An idealapproach is to increase the PFC shared buffer and reduce the bufferreserved for headroom. However, this approach subsequently reducesthe 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 approachenables the system to save the headroom buffer space that is reserved for every PG to guarantee lossless delivery during traffic bursts. Foreach PG, you can assign a lower value for headroom buffer. This headroom buffer is sufficient enough to guarantee lossless behavior as thisbuffer is global and is shared among all the lossless queues.The following table depicts the buffer usage statistics when shared headroom is used:258 Data Center Bridging (DCB)
