Users Guide

• Data Center Bridging Exchange (DCBx) protocol
NOTE: Dell Networking OS supports only the PFC, ETS, and DCBx features in data center bridging.
Priority-Based Flow Control
In a data center network, priority-based flow control (PFC) manages large bursts of one traffic type in multiprotocol links so that it does
not affect other traffic types and no frames are lost due to congestion.
When PFC detects congestion on a queue for a specified priority, it sends a pause frame for the 802.1p priority traffic to the transmitting
device. In this way, PFC ensures that PFC-enabled priority traffic is not dropped by the switch.
PFC enhances the existing 802.3x pause and 802.1p priority capabilities to enable flow control based on 802.1p priorities (classes of
service). Instead of stopping all traffic on a link (as performed by the traditional Ethernet pause mechanism), PFC 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.
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Data Center Bridging (DCB)