Users Guide

• PFC is supported on specified 802.1p priority traffic (dot1p 0 to 7) and is configured per interface.
However, only two lossless queues are supported on an interface: one for Fibre Channel over
Ethernet (FCoE) converged traffic and one for Internet Small Computer System Interface (iSCSI)
storage traffic. Configure the same lossless queues on all ports.
• 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 enabled DCB. When you enable DCB globally, you cannot
simultaneously enable TX and RX on the interface for flow control and link-level flow control is
disabled.
• Buffer space is allocated and de-allocated only when you configure a PFC priority on the port.
• 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.
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Data Center Bridging (DCB)