Concept Guide

• A dynamic threshold handles intermittent trac bursts and varies based on the number of PFC priorities contending for buers, while a

static threshold places an upper limit on the transmit time of a queue after receiving a message to pause a specied priority. PFC trac

is paused only after surpassing both static and dynamic thresholds for the priority specied 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 ow control and link-level ow control is disabled.

• Buer space is allocated and de-allocated only when you congure 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 specied priority.

• By default, PFC is enabled on an interface with no dot1p priorities congured. You can congure 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 conguration, PFC parameters from the peer are

used to congured PFC priorities on the port. If you enable the link-level ow 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-congured PFC input policies are applied. If no PFC dcb-map has

been previously applied, the PFC default setting is used (no priorities congured). 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 buering 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 trac types in multiprotocol (Ethernet, FCoE,

SCSI) links.

ETS allows you to divide trac according to its 802.1p priority into dierent priority groups (trac classes) and congure bandwidth

allocation and queue scheduling for each group to ensure that each trac type is correctly prioritized and receives its required bandwidth.

For example, you can prioritize low-latency storage or server cluster trac in a trac class to receive more bandwidth and restrict best-

eort LAN trac assigned to a dierent trac class.

Although you can congure strict-priority queue scheduling for a priority group, ETS introduces exibility that allows the bandwidth

allocated to each priority group to be dynamically managed according to the amount of LAN, storage, and server trac in a ow. Unused

bandwidth is dynamically allocated to prioritized priority groups. Trac is queued according to its 802.1p priority assignment, while exible

bandwidth allocation and the congured queue-scheduling for a priority group is supported.

The following gure shows how ETS allows you to allocate bandwidth when dierent trac types are classed according to 802.1p priority

and mapped to priority groups.

Data Center Bridging (DCB)