Service Manual

Performing PFC Using DSCP Bits Instead of 802.1p Bits
Priority based Flow Control (PFC) is currently supported on Dell Networking OS for tagged packets based on the packet Dot1p. In
certain data center deployments, VLAN conguration is avoided on the servers and all packets from the servers are untagged. These
packets will carry IP header and can be dierentiated based on the DSCP elds they carry on the server facing switch ports.
Requirement is to classify these untagged packets from the server based on their DSCP and provide PFC treatment.
Dell Networking OS Releases 9.3(0.0) and earlier provide CLI support to specify the priorities for which PFC is enabled on each port.
This feature is applicable only for the tagged packets based on the incoming packet Dot1p and Dot1p based queue classication. This
document will discuss the congurations required to support PFC for untagged packets based on incoming packet DSCP.
For the tagged packets, Queue is selected based on the incoming Packet Dot1p. When PFC frames for a specic priority is received
from the peer switch, the queue corresponding to that Dot1p is halted from scheduling on that port, thus honoring the PFC from the
peer. If a queue is congested due to packets with a specic Dot1p and PFC is enabled for that Dot1p, switch will transit out PFC
frames for that Dot1p. The packet Dot1p to Queue mapping for classication on the ingress must be same as the mapping of Dot1p
to the Queue to be halted on the egress used for PFC honoring. Dell Networking OS ensures that these mappings are identical. This
section discusses the Dell Networking OS congurations needed for above PFC generation and honoring mechanism to work for the
untagged packets.
PRIORITY to PG mapping (PRIO2PG) is on the ingress for each port. By default, all priorities are mapped to PG7. A priority for which
PFC has to be generated is assigned to a PG other than PG7 (say PG6) and buer watermark is set on PG6 so as to generate PFC.
In ingress, the buers are accounted at per PG basis and would indicate the number of the packets that has ingress this port PG but
still queued up in egress pipeline. However, there is no direct mapping between the PG and Queue.
Packet is assigned an internal priority on the ingress pipeline based on the queue to which it is destined. This Internal-priority to
Queue mapping has been modied and enhanced as follows for the device:
Table 19. Priority to Queue Mapping
Internal-
priority
0 1 2 3 4 5 6 7
Queue
2 0 1 3 4 5 6 7
Default dot1p to queue conguration is as follows:
Table 20. Dot1p to Queue Mapping
Packet-
Dot1p
0 1 2 3 4 5 6 7
Queue
2 0 1 3 4 5 6 7
PFC and ETS Conguration Examples
This section contains examples of how to congure and apply DCB policies on an interface.
Using PFC and ETS to Manage Data Center Trac
The following shows examples of using PFC and ETS to manage your data center trac.
In the following example:
• Incoming SAN trac is congured for priority-based ow control.
• Outbound LAN, IPC, and SAN trac is mapped into three ETS priority groups and congured for enhanced trac selection
(bandwidth allocation and scheduling).
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Data Center Bridging (DCB)