Brocade Network Advisor SAN User Manual v11.1x (53-1002167-01, May 2011)

334 Brocade Network Advisor SAN User Manual
53-1002167-01
Enhanced Ethernet features
14
DCB exchange protocol
DCB Exchange (DCBX) protocol allows enhanced Ethernet devices to convey and configure their
DCB capabilities and ensures a consistent configuration across the network. DCBX protocol is used
between data center bridging (DCB) devices, such as a converged network adapter (CNA) and a
FCoE switch, to exchange configuration with directly-connected peers.
NOTE
When DCBX protocol is used, any other LLDP implementation must be disabled on the host systems.
Enhanced Ethernet features
Data Center Bridging (DCB) is a set of IEEE 802 standard Ethernet enhancements that enable Fibre
Channel convergence with Ethernet. The two basic requirements in a lossless Ethernet
environment are Enhanced Transmission Selection (ETS) and priority-based flow control. These
capabilities allow the Fibre Channel frames to run directly over 10 Gbps Ethernet segments without
adversely affecting performance.
Enhanced transmission selection
Enhanced transmission selection (ETS) allows lower priority traffic classes to use available
bandwidth that is not be used by higher priority traffic classes and maximizes the use of available
bandwidth.
ETS allows configuration of bandwidth per priority group.
Priority group ID usage is defined as follows:
• PGID = {0, 7} is used when the priority group is limited for its bandwidth use.
• PGID = {8, 14} is reserved.
• PGID = {15.0 - 15.7} is used for priorities that are not limited for their bandwidth use.
The configured priority group percentage refers to the maximum percentage of available link
bandwidth after PGID 15.0 to 15.7 is serviced, assuming all priority groups are fully subscribed. If
one of the priority groups does not consume its allocated bandwidth, then any unused portion is
available for use by other priority groups.
Priority-based flow control
Priority-based flow control allows the network to selectively pause different classes of traffic and
create lossless lanes for Fibre Channel, while retaining packet drop congestion management for IP
traffic. A high-level pause example follows:
• During periods of heavy congestion, the receive buffers reach high threshold and generate a
pause.
• The pause tells transmission (Tx) queues to stop transmitting.
• After the receive (Rx) buffers reach low threshold, a zero pause is generated.
• The zero pause signals the Tx queues to resume transmitting.