Juniper QFX3000-G QFabric, Руководство по развертыванию, Страница: 97 / 652

QFabric System CoS
When traffic enters and exits the same Node device, CoS works the same as it works on
a standalone QFX3500 switch.
However, when traffic enters a Node device, crosses the Interconnect device, and then
exits a different Node device, CoS is applied differently:
1.
Traffic entering the ingress Node device receives the CoS configured at the Node
ingress (packet classification, congestion notification profile for PFC).
2.
When traffic goes from the ingress Node device to the Interconnect device, the fabric
fc-set CoS is applied as described in the discussion of fabric forwarding class set
scheduling.
3.
When traffic goes from the Interconnect device to the egress Node device, the egress
Node device applies CoS at the egress port (egress queue scheduling, WRED,
IEEE 802.1p or DSCP code-point rewrite).
Support for Flow Control and Lossless Transport Across the Fabric
The Interconnect device incorporates flow control mechanisms to support lossless
transport during periods of congestion on the fabric. To support the priority-based flow
control (PFC) feature on the Node devices, the fabric interfaces use LLFC to support
lossless transport for up to six IEEE 802.1p priorities when the following two configuration
constraints are met:
1.
The IEEE 802.1p priority used for the traffic that requires lossless transport is mapped
to a lossless forwarding class on the Node devices.
2.
The lossless forwarding class must be mapped to a lossless fabric fc-set on the
Interconnect device (
fabric_fcset_noloss1
,
fabric_fcset_noloss2
,
fabric_fcset_noloss3
,
fabric_fcset_noloss4
,
fabric_fcset_noloss5
, or
fabric_fcset_noloss6
).
When traffic meets the two configuration constraints, the fabric propagates the back
pressure from the egress Node device across the fabric to the ingress Node device during
periods of congestion. However, to achieve end-to-end lossless transport across the
switch, you must also configure a congestion notification profile to enable PFC on the
Node device ingress ports.
For all other combinations of IEEE 802.1p priority to forwarding class mapping and all
other combinations of forwarding class to fabric fc-set mapping, the congestion control
mechanism is normal packet drop. For example:
•
Case 1 —If the IEEE 802.1p priority
5
is mapped to the lossless
fcoe
forwarding class,
and the
fcoe
forwarding class is mapped to the
fabric_fcset_noloss1
fabric fc-set, then
the congestion control mechanism is PFC.
•
Case 2 —If the IEEE 802.1p priority
5
is mapped to the lossless
fcoe
forwarding class,
and the
fcoe
forwarding class is mapped to the
fabric_fcset_be
fabric fc-set, then the
congestion control mechanism is packet drop.
71 Copyright © 2015, Juniper Networks, Inc.
Chapter 4: Software Features