Dell S4820T, Configuration Manual

Page: 295 / 1178

Step Task Command Command Mode
Maximum number of lossless queues
supported on an Ethernet port: 2.
Separate priority values with a comma.
Specify a priority range with a dash, for
example: pfc priority 3,5-7
1. You cannot configure PFC using the
pfc priority
command on an
interface on which a DCB map has been
applied or which is already configured
for lossless queues (
pfc no-drop
queues
command).
Configuring Lossless Queues
DCB also supports the manual configuration of lossless queues on an interface when PFC mode is
disabled in a DCB map, apply the map on the interface. The configuration of no-drop queues provides
flexibility for ports on which PFC is not needed, but lossless traffic should egress from the interface.
Configuring no-drop queues is applicable only on the interfaces which do not need PFC.
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:
Dell(conf)#do show qos dot1p-queue-mapping
Dot1p Priority : 0 1 2 3 4 5 6 7 -> On ingress interfaces[Port A and
C] we used the PFC on priority level.
Queue : 0 0 0 1 2 3 3 3 -> On Egress interface[Port B] we
used no-drop queues.
Lossless traffic egresses out the no-drop queues. Ingress 802.1p traffic from PFC-enabled peers is
automatically mapped to the no-drop egress queues.
Data Center Bridging (DCB)
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