Administrator Guide

• If priority group 1 or 2 has free bandwidth, (20 + 30)% of the free bandwidth is
distributed to priority group 3. Priority groups 1 and 2 retain whatever free
bandwidth remains up to the (20+ 30)%.
Strict-priority
groups:
If two priority groups have strict-priority scheduling, traffic assigned from the priority
group with the higher priority-queue number is scheduled first. However, when three
priority groups are used and two groups have strict-priority scheduling (such as groups
1 and 3 in the example), the strict priority group whose traffic is mapped to one queue
takes precedence over the strict priority group whose traffic is mapped to two queues.
Therefore, in this example, scheduling traffic to priority group 1 (mapped to one strict-priority queue) takes
precedence over scheduling traffic to priority group 3 (mapped to two strict-priority queues).
DCBx Operation
The data center bridging exchange protocol (DCBx) is used by DCB devices to exchange configuration
information with directly connected peers using the link layer discovery protocol (LLDP) protocol. DCBx can
detect the misconfiguration of a peer DCB device, and optionally, configure peer DCB devices with DCB
feature settings to ensure consistent operation in a data center network.
DCBx is a prerequisite for using DCB features, such as priority-based flow control (PFC) and enhanced traffic
selection (ETS), to exchange link-level configurations in a converged Ethernet environment. DCBx is also
deployed in topologies that support lossless operation for FCoE or iSCSI traffic. In these scenarios, all network
devices are DCBx-enabled (DCBx is enabled end-to-end).
The following versions of DCBx are supported on an Aggregator: CIN, CEE, and IEEE2.5.
DCBx requires the LLDP to be enabled on all DCB devices.
DCBx Operation
DCBx performs the following operations:
• Discovers DCB configuration (such as PFC and ETS) in a peer device.
• Detects DCB mis-configuration in a peer device; that is, when DCB features are not compatibly
configured on a peer device and the local switch. Mis-configuration detection is feature-specific
because some DCB features support asymmetric configuration.
• Reconfigures a peer device with the DCB configuration from its configuration source if the peer device
is willing to accept configuration.
• Accepts the DCB configuration from a peer if a DCBx port is in “willing” mode to accept a peer’s DCB
settings and then internally propagates the received DCB configuration to its peer ports.
Data Center Bridging (DCB) 292