Setup 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 ow control (PFC) manages large bursts of one trac type in multiprotocol links so that it does not
aect other trac types and no frames are lost due to congestion.
When PFC detects congestion on a queue for a specied priority, it sends a pause frame for the 802.1p priority trac to the transmitting
device. In this way, PFC ensures that PFC-enabled priority trac is not dropped by the switch.
PFC enhances the existing 802.3x pause and 802.1p priority capabilities to enable ow control based on 802.1p priorities (classes of
service). Instead of stopping all trac on a link (as performed by the traditional Ethernet pause mechanism), PFC pauses trac on a link
according to the 802.1p priority set on a trac type. You can create lossless ows for storage and server trac while allowing for loss in
case of LAN trac congestion on the same physical interface.
The following illustration shows how PFC handles trac congestion by pausing the transmission of incoming trac with dot1p priority 4.
Figure 28. Illustration of Trac Congestion
The system supports loading two DCB_Cong les:
• FCoE converged trac with priority 3.
• iSCSI storage trac with priority 4.
In the Dell Networking OS, PFC is implemented as follows:
• PFC is supported on specied 802.1p priority trac (dot1p 0 to 7) and is congured 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 specied priority.
• DCB is disabled on the switch
• By default, PFC is enabled on an interface with no dot1p priorities congured. You can congure 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.
Data Center Bridging (DCB)
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