Setup Guide

Although the system contains 4 MB of space for shared buers, a minimum guaranteed buer is provided to all the internal and external
ports in the system for both unicast and multicast trac. This minimum guaranteed buer reduces the total available shared buer to 3399
KB. This shared buer can be used for lossy and lossless trac.
The default behavior causes up to a maximum of 2656 KB to be used for PFC-related trac. The remaining approximate space of 744 KB
can be used by lossy trac. You can allocate all the remaining 744 KB to lossless PFC queues. If you allocate in such a way, the
performance of lossy trac is reduced and degraded. Although you can allocate a maximum buer size, it is used only if a PFC priority is
congured and applied on the interface.
The number of lossless queues supported on the system is dependent on the availability of total buers for PFC. The default conguration
in the system guarantees a minimum of 9 KB (for 10G) per queue if all the 64 queues are congested. However, modifying the buer
allocation per queue impacts this default behavior.
The default pause threshold size is 9 KB for all interfaces.
This default behavior is impacted if you modify the total buer available for PFC or assign static buer congurations to the individual PFC
queues.
Shared headroom for lossless or PFC packets
In switches that require lossless frame delivery, some xed buer is set aside to absorb any bursty trac that arrives after ow control is
congured (PFC in this case). This extra buer space is called the PG headroom. The additional buer space is reserved for ingress ports
per PG. As the buer is reserved per ingress Port and per PG, the total reserved headroom buer is the sum of the PG headroom buer
reserved for all PGs congured across all ingress ports on the switch.
The PG headroom allocation is done conservatively to guarantee lossless operation in worst case scenarios where huge amounts of bursty
trac arrive at the ingress ports. However, this scheme of allocating headroom buer per PG and per ingress port may result in the
wastage of the reserved PG headroom buer; as, this headroom buer may never be utilized and some of the buer space allocated to PG
headroom is wasted.
To address this issue, Dell EMC Networking OS enables you to congure the shared headroom buer for the entire device. Each PG can
utilize up to the peak headroom congured per PG as part of the buer threshold prole. The traditional threshold for any inight or bursty
trac is set per ingress port and per PG. Retaining the same ingress admission control capabilities, headroom pool can also be used to
manage the headroom buer as a shared resource.
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.
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Data Center Bridging (DCB)