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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.
DellEMC(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
DellEMC(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 interface-type
2. Configure the port queues that will still function as no-drop queues for lossless traffic.
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 configured.
NOTE: Dell EMC Networking OS Behavior: By default, no lossless queues are configured on a port.
A limit of two lossless queues is supported on a port. If the amount of priority traffic that you configure to be paused exceeds
the two lossless queues, an error message displays.
Configuring PFC in a DCB Map
A switch supports the use of a DCB map in which you configure priority-based flow control (PFC) setting. To configure PFC
parameters, you must apply a DCB map on an interface.
PFC Configuration Notes
PFC provides flow control based on the 802.1p priorities in a converged Ethernet traffic 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 traffic
transmission for specified priorities (CoS values) without impacting other priority classes. Different traffic types are assigned to
different priority classes.
When traffic congestion occurs, PFC sends a pause frame to a peer device with the CoS priority values of the traffic that
needs to be stopped. DCBx provides the link-level exchange of PFC parameters between peer devices. PFC allows network
administrators to create zero-loss links for SAN traffic that requires no-drop service, while at the same time retaining packet-
drop congestion management for LAN traffic.
On a switch, PFC is enabled by default on Ethernet ports (pfc mode on command). You can configure PFC parameters
using a DCB map or the pfc priority command in Interface configuration mode. For more information, see Configuring
Priority-Based Flow Control.
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Data Center Bridging (DCB)