Dell Networking S6000 Uživatelský manuál stáhnout pdf (Strana 272)

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policy map to forward the matched DSCP packet to that queue. PFC frames gets generated with PFC

priority associated with the queue when the queue gets congested.

Operations on Untagged Packets

The below is example for enabling PFC for priority 2 for tagged packets. Priority (Packet Dot1p) 2 will be

mapped to PG6 on PRIO2PG setting. All other Priorities for which PFC is not enabled are mapped to

default PG – PG7.

Classification rules on ingress (Ingress FP CAM region) matches incoming packet-dot1p and assigns an

internal priority (to select queue as per Table 1 and Table 2).

The internal Priority assigned for the packet by Ingress FP is used by the memory management unit

(MMU) to assign the packet to right queue by indexing the internal-priority to queue map table (TABLE 1)

in hardware.

PRIO2COS setting for honoring the PFC protocol packets from the Peer switches is as per above Packet-

Dot1p->queue table (Table 2).

The packets that come in with packet-dot1p 2 alone will be assigned to PG6 on ingress.

The packets that come in with packet-dot1p 2 alone will use Q1 (as per dot1p to Queue classification –

Table 2) on the egress port.

• When Peer sends a PFC message for Priority 2, based on above PRIO2COS table (TABLE 2), Queue 1 is

halted.

• Queue 1 starts buffering the packets with Dot1p 2. This causes PG6 buffer counter to increase on the

ingress, since P-dot1p 2 is mapped to PG6.

• As the PG6 watermark threshold is reached, PFC will be generated for dot1p 2.

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 configuration is avoided on the servers

and all packets from the servers are untagged. These packets will carry IP header and can be

differentiated based on the DSCP fields 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 classification. This document will discuss the configurations

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

specific 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 specific Dot1p and PFC is enabled for that Dot1p, switch will transit out PFC frames for that Dot1p.

The packet Dot1p to Queue mapping for classification 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 configurations needed for

above PFC generation and honoring mechanism to work for the untagged packets.

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