Dell S4820T, Configuration Manual

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NOTE: You cannot enable PFC and link-level flow control at the same time on an interface.
Configuring Lossless Queues
DCB also supports the manual configuration of lossless queues on an interface when PFC mode is turned
off.
Prerequisite: A DCB with PFC configuration is applied to the interface with the following conditions:
• PFC mode is off (no pfc mode on).
• No PFC priority classes are configured (
no pfc priority priority-range
).
Example:
Port A —> Port B
Port C —> Port B
PFC no-drop queues are configured for queues 1, 2 on Port B. PFC capability is enabled on priorities 3, 4
on PORT A and C.
Port B acting as Egress
During the congestion, [traffic pump on priorities 3 and 4 from PORT A and PORT C is at full line rate],
PORT A and C send out the PFCs to rate the traffic limit. Egress drops are not observed on Port B since
traffic flow on priorities is mapped to loss less queues.
Port B acting as Ingress
If the traffic congestion is on PORT B , Egress DROP is on PORT A or C, as the PFC is not enabled on
PORT B.
Refer the following configuration for queue to dot1p mapping:
NOTE: Although, each port on the S4810, S4820T, and S5000 devices support 8 QoS queues, you
can configure only 4 QoS queues (0-3) to manage data traffic. The remaining 4 queues (4-7) are
reserved for control traffic.
Dell(conf)#do show qos dot1p-queue-mapping
Dot1p Priority : 0 1 2 3 4 5 6 7
Queue : 0 0 0 1 2 3 3 3
Dell(conf)#
The configuration of no-drop queues provides flexibility for ports on which PFC is not needed but
lossless traffic should egress from the interface.
Lossless traffic egresses out the no-drop queues. Ingress dot1p traffic from PFC-enabled interfaces is
automatically mapped to the no-drop egress queues.
1. Enter INTERFACE Configuration mode.
CONFIGURATION mode
interface type slot/port
2. Configure the port queues that will still function as no-drop queues for lossless traffic.
INTERFACE mode
Data Center Bridging (DCB)
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