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● PFC delay constraints place an upper limit on the transmit time of a queue after receiving a message to pause a specified
priority.
● 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 link layer discovery protocol (LLDP) type, length, value (TLV) to determine
current policies, such as PFC support and enhanced transmission selection (ETS) BW allocation.
○ If the negotiation succeeds and the port is in DCBX Willing mode to receive a peer configuration, PFC parameters from
the peer are used to configured PFC priorities on the port. If you enable the link-level flow control mechanism on the
interface, DCBX negotiation with a peer is not performed.
○ If the negotiation fails and PFC is enabled on the port, any user-configured PFC input policies are applied. If no PFC
dcb-map has been previously applied, the PFC default setting is used (no priorities configured). If you do not enable PFC
on an interface, you can enable the 802.3x link-level pause function. By default, the link-level pause is disabled, when you
disable DCBx and PFC. If no PFC dcb-map has been applied on the interface, the default PFC settings are used.
● PFC supports buffering to receive data that continues to arrive on an interface while the remote system reacts to the PFC
operation.
● PFC uses the DCB MIB IEEE802.1azd2.5 and the PFC MIB IEEE802.1bb-d2.2.
If DCBx negotiation is not successful (for example, due to a version or TLV mismatch), DCBx is disabled and you cannot enable
PFC or ETS.
Enhanced Transmission Selection
Enhanced transmission selection (ETS) supports optimized bandwidth allocation between traffic types in multiprotocol
(Ethernet, FCoE, SCSI) links.
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.
Although you can configure strict-priority queue scheduling for a priority group, ETS introduces flexibility that allows the
bandwidth allocated to each priority group to be dynamically managed according to the amount of LAN, storage, and server
traffic in a flow. Unused bandwidth is dynamically allocated to prioritized priority groups. Traffic is queued according to its
802.1p priority assignment, while flexible bandwidth allocation and the configured queue-scheduling for a priority group is
supported.
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.
Figure 29. Enhanced Transmission Selection
The following table lists the traffic groupings ETS uses to select multiprotocol traffic for transmission.
Data Center Bridging (DCB)
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