Administrator Guide

pauses traffic on a link according to the 802.1p priority set on a traffic type. You can create lossless flows for storage and server
traffic while allowing for loss in case of LAN traffic congestion on the same physical interface.
The following illustration shows how PFC handles traffic congestion by pausing the transmission of incoming traffic with dot1p
priority 4.
Figure 28. Illustration of Traffic Congestion
The system supports loading two DCB_Config files:
● FCoE converged traffic with priority 3.
● iSCSI storage traffic with priority 4.
In the Dell Networking OS, PFC is implemented as follows:
● PFC is supported on specified 802.1p priority traffic (dot1p 0 to 7) and is configured per interface. However, only 4 lossless
queues are supported on an interface.
● PFC delay constraints place an upper limit on the transmit time of a queue after receiving a message to pause a specified
priority.
● DCB is disabled on the switch
● By default, PFC is enabled on an interface with no dot1p priorities configured. You can configure the PFC priorities if the
switch negotiates with a remote peer using DCBx
● During DCBx negotiation with a remote peer:
○ DCBx communicates with the remote peer by LLDP TLV to determine current policies, such as PFC support and ETS
bandwidth allocation.
○ If DCBx negotiation is not successful (for example, a version or TLV mismatch), DCBx is disabled and PFC or ETS cannot
be enabled.
○ PFC uses DCB MIB IEEE 802.1azd2.5 and PFC MIB IEEE 802.1bb-d2.2.
● A dynamic threshold handles intermittent traffic bursts and varies based on the number of PFC priorities contending for
buffers, while a static threshold places an upper limit on the transmit time of a queue after receiving a message to pause a
specified priority. PFC traffic is paused only after surpassing both static and dynamic thresholds for the priority specified for
the port.
● By default, PFC is enabled when you enable DCB. If you have not loaded FCoE_DCB_Config and iSCSI_DCB_Config, DCB is
disabled. When you enable DCB globally, you cannot simultaneously enable link-level flow control.
● Buffer space is allocated and de-allocated only when you configure a PFC priority on the port.
Enhanced Transmission Selection
Enhanced transmission selection (ETS) supports optimized bandwidth allocation between traffic types in multiprotocol
(Ethernet, FCoE, SCSI) links. By default, ETS is disabled.
ETS allows you to divide traffic according to its 802.1p priority into different priority groups (traffic classes) and configure
bandwidth allocation and queue scheduling for each group to ensure that each traffic type is correctly prioritized and receives
its required bandwidth. For example, you can prioritize low-latency storage or server cluster traffic in a traffic class to receive
more bandwidth and restrict best-effort LAN traffic assigned to a different traffic class.
The following figure shows how ETS allows you to allocate bandwidth when different traffic types are classed according to
802.1p priority and mapped to priority groups.
Data Center Bridging (DCB)
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