Users Guide

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:
NOTE: Although, each port on the S4810, S4820T, and S5000 devices support 8 QoS queues, you can congure only 4
QoS queues (0-3) to manage data trac. The remaining 4 queues (4-7) are reserved for control trac.
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 conguration of no-drop queues provides exibility for ports on which PFC is not needed but lossless trac should egress from
the interface.
Lossless trac egresses out the no-drop queues. Ingress dot1p trac from PFC-enabled interfaces is automatically mapped to the
no-drop egress queues.
1. Enter INTERFACE Conguration mode.
CONFIGURATION mode
interface type slot/port
2. Congure the port queues that will still function as no-drop queues for lossless trac.
INTERFACE mode
pfc no-drop queues queue-range
For the dot1p-queue assignments, refer to the dot1p Priority-Queue Assignment table.
The maximum number of lossless queues globally supported on the switch is two.
The range is from 0 to 3. Separate the queue values with a comma; specify a priority range with a dash; for example, pfc no-
drop queues 1,3
or pfc no-drop queues 2-3.
The default: No lossless queues are congured.
NOTE: Dell Networking OS Behavior: By default, no lossless queues are congured on a port.
A limit of two lossless queues is supported on a port. If the amount of priority trac that you congure to be paused exceeds the
two lossless queues, an error message displays.
Conguring PFC in a DCB Map
An S4810 switch supports the use of a DCB map in which you congure priority-based ow control (PFC) setting. To congure PFC
parameters, you must apply a DCB map on
an S4810 interface. This functionality is supported on the S4810 platform.
PFC Conguration Notes
PFC provides ow control based on the 802.1p priorities in a converged Ethernet trac that is received on an interface and is
enabled by default when you enable DCB. As an enhancement to the existing Ethernet pause functionality, PFC stops trac
transmission for specied priorities (CoS values) without impacting other priority classes. Dierent trac types are assigned to
dierent priority classes.
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Data Center Bridging (DCB)