Qlogic Storage Networking (Unified Fabric Pilot), Installation Manual

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B–Data Center Bridging Technology
Data Center Bridging (DCB)
B-2 51031-00 A
Enhanced Transmission Selection (ETS) protocol addresses the bandwidth
allocation issues among various traffic classes to maximize bandwidth usage. The
IEEE 802.1Qaz standard specifies the protocol to support allocation of bandwidth
amongst priority groups. ETS allows each node to control bandwidth per priority
group. When the actual load in a priority group does not use its allocated
bandwidth, ETS allows other priority groups to use the available bandwidth. The
bandwidth-allocation priorities allow the sharing of bandwidth between traffic
loads, while satisfying the strict priority mechanisms already defined in IEEE
802.1Q that require minimum latency.
Bandwidth allocation is achieved as part of a negotiation process with link
peers—this is called DCB Capability eXchange protocol (DCBX). It provides a
mechanism for Ethernet devices (bridges, end stations) to detect the DCB
capability of a peer device. It also allows configuration and distribution of ETS
parameters from one node to another.
ETS and DCBX simplify the management of DCB nodes significantly, especially
when deployed end-to-end in a converged data center. The DCBX protocol uses
Link Layer Discovery Protocol (LLDP) defined by IEEE 802.1AB to exchange and
discover DCB capabilities.
Priority Flow Control
A fundamental requirement for a high performance storage network is guaranteed
data delivery. This requirement must be satisfied to transport critical storage data
on a converged Ethernet network with minimum latency. Another critical
enhancement to conventional Ethernet is lossless Ethernet. IEEE 802.3X PAUSE
defines how to pause link traffic at a congestion point to avoid packet drop. IEEE
802.1Qbb defines Priority Flow Control (PFC), which is based on IEEE 802.3X
PAUSE and provides greater control of traffic flow. PFC eliminates lost frames
caused by congestion. PFC enables the pausing of less sensitive data classes,
while not affecting traditional LAN protocols operating through different priority
classes.
Figure B-1 shows how PFC works in a converged traffic scenario.