Juniper QFX3000-G QFabric, Руководство по развертыванию, Страница: 98 / 652

•
Case 3 —If the IEEE 802.1p priority
5
is mapped to the lossless
no-loss
forwarding class,
and the
no-loss
forwarding class is mapped to the
fabric_fcset_noloss2
fabric fc-set,
then the congestion control mechanism is PFC.
• Case 4 —If the IEEE 802.1p priority 5 is mapped to the lossless no-loss forwarding class,
and the no-loss forwarding class is mapped to the fabric_fcset_be fabric fc-set, then
the congestion control mechanism is packet drop.
• Case 5 —If the IEEE 802.1p priority 5 is mapped to the best-effort forwarding class, and
the best-effort forwarding class is mapped to the fabric_fcset_be fabric fc-set, then
the congestion control mechanism is packet drop.
•
Case 6 —If the IEEE 802.1p priority
5
is mapped to the
best-effort
forwarding class, and
the
best-effort
forwarding class is mapped to the
fabric_fcset_noloss1
fabric fc-set,
then the congestion control mechanism is packet drop.
NOTE: Lossless transport across the fabric also must meet the following
two conditions:
1.
The maximum cable length between the Node device and the Interconnect
device is a 150 meters of fiber cable.
2.
The maximum frame size is 9216 bytes.
If the MTU is 9216 KB, in some cases the QFabric system supports only five
lossless forwarding classes instead of six lossless forwarding classes because
of headroom buffer limitations.
The number of IEEE 802.1p priorities (forwarding classes) the QFabric system can support
for lossless transport across the Interconnect device fabric depends on several factors:
•
Approximate fiber cable length—The longer the fiber cable that connects Node device
fabric (FTE) ports to the Interconnect device fabric ports, the more data the connected
ports need to buffer when a pause is asserted. (The longer the fiber cable, the more
frames are traversing the cable when a pause is asserted. Each port must be able to
store all of the “in transit” frames in the buffer to preserve lossless behavior and avoid
dropping frames.)
• MTU size—The larger the maximum frame sizes the buffer must hold, the fewer frames
the buffer can hold. The larger the MTU size, the more buffer space each frame
consumes.
•
Total number of Node device fabric ports connected to the Interconnect device—The
higher the number of connected fabric ports, the more headroom buffer space the
Node device needs on those fabric ports to support the lossless flows that traverse
the Interconnect device. Because more buffer space is used on the Node device fabric
ports, less buffer space is available for the Node device access ports, and a lower total
number of lossless flows are supported.
Copyright © 2015, Juniper Networks, Inc. 72
QFX3000-G QFabric System Deployment Guide