Administrator Guide

Port B acting as Egress
During the congestion, [trac 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 trac limit. Egress drops are not observed on Port B since trac ow on priorities is mapped to loss less queues.
Port B acting as Ingress
If the trac 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 conguration 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 trac egresses out the no-drop queues. Ingress 802.1p trac from PFC-enabled peers is automatically mapped to the no-drop
egress queues.
When conguring lossless queues on a port interface, consider the following points:
• By default, no lossless queues are congured on a port.
• A limit of two lossless queues is supported on a port. If the number of lossless queues congured exceeds the maximum supported limit
per port (two), an error message is displayed. Recongure the value to a smaller number of queues.
• If you congure lossless queues on an interface that already has a DCB map with PFC enabled (pfc on), an error message is displayed.
Table 18.
Conguring Lossless Queues on a Port Interface
Step Task Command Command Mode
1 Enter INTERFACE Conguration mode.
interface
{tengigabitEthernet slot/
port [/subport] |
fortygigabitEthernet slot/
port}
CONFIGURATION
2
Open a DCB map and enter DCB map conguration mode.
dcb-map name
INTERFACE
3
Disable PFC.
no pfc mode on
DCB MAP
4
Return to interface conguration mode.
exit
DCB MAP
5
Apply the DCB map, created to disable the PFC operation,
on the interface
dcb-map {name | default}
INTERFACE
6
Congure the port queues that still function as no-drop
queues for lossless trac. For the dot1p-queue
assignments.
The maximum number of lossless queues globally supported
on a port is 2.
You cannot congure PFC no-drop queues on an interface
on which a DCB map with PFC enabled has been applied,
or which is already congured for PFC using the pfc
priority command.
Range: 0-3. Separate queue values with a comma; specify a
priority range with a dash; for example: pfc no-drop queues
pfc no-drop
queuesqueue-range
INTERFACE
Data Center Bridging (DCB) 263