Dell PowerEdge FX2/FX2s - Manual (Page 37)

NOTE: Dell Networking does not recommend mapping all ingress traffic to a single queue when
using PFC and ETS. However, Dell Networking does recommend using Ingress traffic classification
using the service-class dynamic dot1p command (honor dot1p) on all DCB-enabled
interfaces. If you use L2 class maps to map dot1p priority traffic to egress queues, take into account
the default dot1p-queue assignments in the following table and the maximum number of two
lossless queues supported on a port.
Although the system allows you to change the default dot1p priority-queue assignments, DCB
policies applied to an interface may become invalid if you reconfigure dot1p-queue mapping. If the
configured dcb-map remains valid, the change in the dot1p-queue assignment is allowed. For DCB
ETS enabled interfaces, traffic destined to queue that is not mapped to any dot1p priority are
dropped.
dot1p Value in the
Incoming Frame Egress Queue Assignment
0 0
1 0
2 0
3 1
4 2
5 3
6 3
7 3
How Priority-Based Flow Control is Implemented
Priority-based flow control provides a flow control mechanism based on the 802.1p priorities in
converged Ethernet traffic received on an interface and is enabled by default. As an enhancement to the
existing Ethernet pause mechanism, 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 creates zero-loss links for SAN traffic that requires no-drop service, while at the same
time retaining packet-drop congestion management for LAN traffic.
PFC is implemented on an Aggregator as follows:
• If DCB is enabled, as soon as a dcb-map with PFC is applied on an interface, DCBx starts exchanging
information with PFC-enabled peers. The IEEE802.1Qbb, CEE and CIN versions of PFC TLV are
supported. DCBxalso validates PFC configurations received in TLVs from peer devices.
• To achieve complete lossless handling of traffic, enable PFC operation on ingress port traffic and on
all DCB egress port traffic.
• All 802.1p priorities are enabled for PFC. Queues to which PFC priority traffic is mapped are lossless by
default. Traffic may be interrupted due to an interface flap (going down and coming up).
• For PFC to be applied on an Aggregator port, the auto-configured priority traffic must be supported
by a PFC peer (as detected by DCBx).
• A dcb-map for PFC applied to an interface may become invalid if dot1p-queue mapping is
reconfigured. This situation occurs when the new dot1p-queue assignment exceeds the maximum
Data Center Bridging (DCB) 37