User Manual

• 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.

Configuring Priority-Based Flow Control

PFC provides a flow control mechanism based on the 802.1p priorities in converged Ethernet traffic

received on an interface and is enabled by default when you enable DCB.

As an enhancement to the existing Ethernet pause mechanism, PFC stops traffic transmission for

specified priorities (Class of Service (CoS) values) without impacting other priority classes. Different traffic

types are assigned to different priority classes.

When traffic congestion occurs, PFC sends a pause frame to a peer device with the CoS priority values of

the traffic that is to be stopped. Data Center Bridging Exchange protocol (DCBx) provides the link-level

exchange of PFC parameters between peer devices. PFC allows network administrators to create zero-

loss links for Storage Area Network (SAN) traffic that requires no-drop service, while retaining packet-

drop congestion management for Local Area Network (LAN) traffic.

To ensure complete no-drop service, apply the same dcb-map on all PFC and ETS enabled interfaces.

1. Create a DCB map to apply priority based flow control or enhanced transmission selection for

specified priority groups and priorities.

CONFIGURATION mode

dcb-map map-name

The maximum is 32 alphanumeric characters.

2. Configure the priority group with PGID, bandwidth percentage or strict priority for ETS and PFC

mode.

DCB-MAP mode

priority-group pg_num [bandwidth percentage | strict-priority] pfc [on |

off]

pg_num range is from 0 to 7.

bandwidth percentage range is from 1 to 100.

Data Center Bridging (DCB)