Owner`s manual

52 | Data Center Bridging (DCB)
www.dell.com | support.dell.com
• You can enable link-level flow control on the interface (refer to Ethernet Pause Frames). To delete the
input policy, first disable link-level flow control. PFC is then automatically enabled on the interface
because an interface is by default PFC-enabled.
• PFC still allows you to configure lossless queues on a port to ensure no-drop handling of lossless
traffic (refer to Configuring Lossless Queues).
You cannot enable PFC and link-level flow control at the same time on an interface.
When you apply an input policy to an interface, an error message displays if:
• The PFC dot1p priorities result in more than two lossless port queues globally on the switch.•
• You already enabled link-level flow control. You cannot enable PFC and link-level flow control at the
same time on an interface.•
• In a switch stack, configure all stacked ports with the same PFC configuration.
A DCB input policy for PFC applied to an interface may become invalid if you reconfigure dot1p-queue
mapping (refer to the Create Input Policy Maps section in the Quality of Service (QoS) chapter). This
situation occurs when the new dot1p-queue assignment exceeds the maximum number (2) of lossless
queues supported globally on the switch. In this case, all PFC configurations received from PFC-enabled
peers are removed and resynchronized with the peer devices.
Traffic may be interrupted when you reconfigure PFC no-drop priorities in an input policy or reapply the
policy to an interface.
Enhanced Transmission Selection
Enhanced transmission selection (ETS) supports optimized bandwidth allocation between traffic types in
multiprotocol (Ethernet, FCoE, SCSI) links.
ETS allows you to divide traffic according to its 802.1p priority into different priority groups (traffic
classes) and configure bandwidth allocation and queue scheduling for each group to ensure that each traffic
type is correctly prioritized and receives its required bandwidth. For example, you can prioritize
low-latency storage or server cluster traffic in a traffic class to receive more bandwidth and restrict
best-effort LAN traffic assigned to a different traffic class.
Although you can configure strict-priority queue scheduling for a priority group, ETS introduces flexibility
that allows the bandwidth allocated to each priority group to be dynamically managed according to the
amount of LAN, storage, and server traffic in a flow. Unused bandwidth is dynamically allocated to
prioritized priority groups. Traffic is queued according to its 802.1p priority assignment, while flexible
bandwidth allocation and the configured queue-scheduling for a priority group is supported.
Figure 5-2 shows how ETS allows you to allocate bandwidth when different traffic types are classed
according to 802.1p priority and mapped to priority groups.