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.
When configuring lossless queues on a port interface, consider the following points:
• By default, no lossless queues are configured on a port.
• A limit of two lossless queues is supported on a port. If the number of lossless queues configured
exceeds the maximum supported limit per port (two), an error message is displayed. Reconfigure the
value to a smaller number of queues.
• If you configure lossless queues on an interface that already has a DCB map with PFC enabled (pfc on),
an error message is displayed.
Table 19. Configuring Lossless Queues on a Port Interface
Step Task Command Command Mode
1 Enter INTERFACE Configuration mode.
interface
{tengigabitEthernet
slot/port [/
subport] |
fortygigabitEthernet
slot/port}
CONFIGURATION
2
Open a DCB map and enter DCB map
configuration mode.
dcb-map name
INTERFACE
3
Disable PFC.
no pfc mode on
DCB MAP
4
Return to interface configuration mode.
exit
DCB MAP
5
Apply the DCB map, created to disable the PFC
operation, on the interface
dcb-map {name |
default}
INTERFACE
6
Configure the port queues that still function as
no-drop queues for lossless traffic. For the
dot1p-queue assignments.
pfc no-drop
queuesqueue-range
INTERFACE
Data Center Bridging (DCB) 307
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