Service Manual

• 802.1Qaz — Enhanced Transmission Selection (ETS)
• 802.1Qau — Congestion Notication
• 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 trac type in multiprotocol links so that it
does not aect other trac types and no frames are lost due to congestion.
When PFC detects congestion on a queue for a specied priority, it sends a pause frame for the 802.1p priority trac to the
transmitting device. In this way, PFC ensures that PFC-enabled priority trac 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 trac on a link (as performed by the traditional Ethernet pause mechanism), PFC pauses trac on a
link according to the 802.1p priority set on a trac type. You can create lossless ows for storage and server trac while allowing for
loss in case of LAN trac congestion on the same physical interface.
The following illustration shows how PFC handles trac congestion by pausing the transmission of incoming trac with dot1p
priority 4.
The system supports loading two DCB_Cong les:
• FCoE converged trac with priority 3.
• iSCSI storage trac with priority 4.
In the Dell Networking OS, PFC is implemented as follows:
• PFC is supported on specied 802.1p priority trac (dot1p 0 to 7) and is congured per interface. However, only two lossless
queues are supported on an interface: one for FCoE converged trac with priority 3 and one for iSCSI storage trac with
priority 4. You must congure the same lossless queues on all ports.
• 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 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.
• PFC supports buering to receive data that continues to arrive on an interface while the remote system reacts to the PFC
operation.
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Data Center Bridging (DCB)