Supported Platforms
Overview of CoS Changes Introduced in Junos OS Release 12.2
Junos OS Release 12.2 introduces some changes to class-of-service (CoS) functionality and to the CoS default values. This overview summarizes the changes, which other documents describe in detail.
This topic describes the following changes in CoS default values and behavior:
Lossless Forwarding Classes (fcoe and no-loss)
The way the QFX Series handles lossless forwarding classes (the fcoe and no-loss forwarding classes) changes in Junos OS Release 12.2. In Junos OS Release 12.2 and in earlier releases, by default, the fcoe and no-loss forwarding classes are mapped to output queue 3 and output queue 4, respectively. These are the only two forwarding classes (and the only two queues) that support lossless transport.
In earlier releases, explicitly setting the lossless fcoe and no-loss forwarding classes resulted in the same CoS behavior as using the default configuration. However, in Junos OS Release 12.2, the behavior when you explicitly configure the lossless forwarding classes differs from the behavior when you use the default forwarding classes.
 | Note: The default behavior differs from the explicit configuration behavior even if the explicit configuration is exactly the same as the default configuration. |
If you use the default forwarding class settings for the lossless queues (the configuration does not include explicit setting of the fcoe or the no-loss forwarding classes), then the fcoe and no-loss queues behave as lossless queues. When you upgrade to Junos OS Release 12.2, traffic assigned to the fcoe and no-loss queues continues to be treated as lossless traffic.
If your configuration explicitly sets the fcoe or the no-loss forwarding class (set class-of-service forwarding-classes class class-name queue-num queue-number), after you upgrade to Junos OS Release 12.2, those queues do not receive lossless treatment and behave as lossy (best-effort) queues. To retain lossless treatment of the fcoe and no-loss queues, delete the explicit lossless forwarding class configuration before you upgrade to Junos OS Release 12.2.
 | Caution: If you explicitly configured the fcoe or the no-loss forwarding class, and you upgrade to Junos OS Release 12.2, the system does not return an upgrade error or a commit error, or a generate a syslog message, to notify you that these forwarding classes are no longer lossless. Traffic mapped to these forwarding classes is not treated as lossless traffic until you remove the explicit forwarding class configuration. |
Default MTU for Headroom Buffer Calculation for Lossless Forwarding Classes
The default maximum transmission unit (MTU) the system uses for buffer headroom calculation is 2500 bytes for traffic classified into the fcoe forwarding class or the no-loss forwarding class.
In Junos OS Release 12.2, the default MTU used for buffer headroom calculation for the fcoe and no-loss forwarding classes remains 2500 bytes. However, if the buffer is filled, in Junos OS Release 12.2 you might experience commit failures.
CoS for Layer 3 Physical Interfaces
Before Junos OS Release 12.2, the QFX Series supported only Layer 2 CoS. Junos OS Release 12.2 introduces CoS support for Layer 3 traffic at the physical interface level.
If a physical Layer 3 interface has at least one logical interface configured on it, you can configure Layer 3 CoS for the physical interface. The CoS configured on the physical interface applies to all of the logical Layer 3 interfaces on that physical interface. The system does not support Layer 3 CoS configuration on individual Layer 3 logical interfaces.
DSCP IPv6 Classifiers and Rewrite Rules
Junos OS Release 12.2 introduces support for DSCP IPv6 classifiers and rewrite rules. The existing DSCP IP default classifier is now also the DSCP IPv6 default classifier.
You can configure and apply DSCP IPv6 classifiers and DSCP IPv6 rewrite rules to Layer 2 logical interfaces and to Layer 3 physical interfaces.
| Note: DSCP IPv6 classifiers are not supported for multidestination (multicast, broadcast, and destination lookup fail) traffic. |
