Administrator Guide

NOTE: Dell Networking does not recommend mapping all ingress trac to a single queue when using PFC and ETS.
However, Dell Networking does recommend using Ingress trac classication using the service-class dynamic
dot1p command (honor dot1p) on all DCB-enabled interfaces. If you use L2 class maps to map dot1p priority trac 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 recongure dot1p-queue mapping. If the congured dcb-map remains valid, the change in the dot1p-
queue assignment is allowed. For DCB ETS enabled interfaces, trac 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 ow control provides a ow control mechanism based on the 802.1p priorities in converged Ethernet trac received
on an interface and is enabled by default. As an enhancement to the existing Ethernet pause mechanism, 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.
When trac congestion occurs, PFC sends a pause frame to a peer device with the CoS priority values of the trac that needs to
be stopped. DCBx provides the link-level exchange of PFC parameters between peer devices. PFC creates zero-loss links for SAN
trac that requires no-drop service, while at the same time retaining packet-drop congestion management for LAN trac.
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. DCBx also validates PFC congurations
received in TLVs from peer devices.
• To achieve complete lossless handling of trac, enable PFC operation on ingress port trac and on all DCB egress port trac.
• All 802.1p priorities are enabled for PFC. Queues to which PFC priority trac is mapped are lossless by default. Trac may be
interrupted due to an interface ap (going down and coming up).
• For PFC to be applied on an Aggregator port, the auto-congured priority trac 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 recongured. This situation occurs
when the new dot1p-queue assignment exceeds the maximum number (2) of lossless queues supported globally on the switch.
In this case, all PFC congurations received from PFC-enabled peers are removed and re-synchronized with the peer devices.
• Dell Networking OS does not support MACsec Bypass Capability (MBC).
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Data Center Bridging (DCB)