Understanding CoS Forwarding Classes
Forwarding classes group traffic and assign the traffic to output queues. Each forwarding class correlates to an output queue. The result of classification is the identification of an output queue for a particular packet.
A classifier must associate each packet with one of the following five default forwarding classes or with a user-configured forwarding class in order to assign an output queue to the packet:
- fcoe—Guaranteed delivery for Fibre Channel over Ethernet (FCoE) traffic.
- no-loss—Guaranteed delivery for TCP lossless traffic.
- best-effort—Provides best-effort delivery without a service profile. Loss priority is typically not carried in a class-of-service (CoS) value.
- network-control—Supports protocol control and is typically high priority.
- mcast—Provides no service profile for multidestination (multicast, broadcast, and destination lookup fail) packets.
The switch supports up to 12 forwarding classes, thus enabling flexible, differentiated, packet classification. For example, you can configure multiple classes of best-effort traffic such as best-effort, best-effort1, and best-effort2.
The switch supports up to 12 output queues: 8 output queues for unicast traffic (queues 0 through 7) and 4 output queues for multidestination traffic (queues 8 through 11). Forwarding classes mapped to unicast queues are associated with unicast traffic, and forwarding classes mapped to multidestination queues are associated with multidestination traffic. You cannot map unicast and multidestination traffic to the same queue. You cannot map a strict-high priority queue to a multidestination forwarding class (queues 8 through 11 do not support strict-high priority configuration).
Default Forwarding Classes
Table 1 shows the four default forwarding classes defined for unicast traffic, and Table 2 shows the four default forwarding classes defined for multicast traffic.
If desired, you can rename the forwarding classes associated with the queues supported on your switch. Assigning a new class name to an output queue does not alter the default classification or scheduling that is applicable to that queue. CoS configurations can be quite complicated, so unless it is required by your scenario, we recommend that you not alter the default class names or queue number associations.
Table 1: Default Forwarding Classes for Unicast Packets
Forwarding Class Name | Default Queue Mapping | Comments |
|---|---|---|
best-effort (be) | 0 | The software does not apply any special CoS handling to packets with 000000 in the DiffServ field. This is a backward compatibility feature. These packets are usually dropped under congested network conditions. |
fcoe | 3 | The software delivers assured bandwidth, low loss, low delay, and low delay variation (jitter) end to end for packets in this service class. The software accepts excess traffic in this class, but in contrast to the assured forwarding class, the out-of-profile expedited-forwarding class packets can be forwarded out of sequence or dropped. Note: By convention, deployments with converged server access typically use IEEE 802.1p priority 3 (011) for FCoE traffic. The default mapping of the fcoe forwarding class is to queue 3. Apply priority-based flow control (PFC) to the entire FCoE data path to configure the end-to-end lossless behavior that FCoE requires. Always use priority 3 for FCoE traffic. |
no-loss | 4 | The software offers a high level of assurance that the packets are delivered as long as the packet flow from the customer stays within a certain service profile that you define. The software accepts excess traffic, but it applies a tail-drop profile to determine if the excess packets are dropped and not forwarded. Up to two drop probabilities (low and high) are defined for this service class. |
network-control (nc) | 7 | The software delivers packets in this service class with a high priority. (These packets are not delay-sensitive.) Typically, these packets represent routing protocol hello or keepalive messages. Because loss of these packets jeopardizes proper network operation, packet delay is preferable to packet discard. |
Table 2: Default Forwarding Classes for Multicast Packets
Forwarding Class Name | Default Queue Mapping | Comments |
|---|---|---|
mcast | 8 | The software does not apply any special CoS handling to the multidestination packets. These packets are usually dropped under congested network conditions. |
 | Note: Mirrored traffic is always sent to the queue that corresponds to the multidestination forwarding class. The switched copy of the mirrored traffic is forwarded with the priority determined by the behavior aggregate classification process. |
Forwarding Class Configuration Rules
Take the following rules into account when you configure forwarding classes:
Queue Assignment Rules
The following rules govern queue assignment:
- CoS configurations that specify more queues than the switch can support are not accepted. The commit operation fails with a detailed message that states the total number of queues available.
- All default CoS configurations are based on queue number. The name of the forwarding class that appears in the default configuration is the forwarding class currently associated with that queue.
- Only unicast forwarding classes can be mapped to unicast queues (0 through 7), and only multidestination forwarding classes can be mapped to multidestination queues (8 through 11).
- Strict-high priority queues cannot be mapped to multidestination forwarding classes. (Strict-high priority traffic cannot be mapped to queues 8 through 11).
Scheduling Rules
When you define a forwarding class that is used on the switch (the behavior aggregate classifier has a forwarding class and you expect traffic for the forwarding class), you must also define a scheduling policy for the forwarding class. Defining a scheduling policy means:
- Mapping a scheduler to the forwarding class in a scheduler map
- Including the forwarding class in a forwarding class set
- Associating the scheduler map with a traffic control profile
- Attaching the traffic control profile to a forwarding class set and an interface
Rewrite Rules
On each physical interface, either all forwarding classes that are being used on the interface must have rewrite rules configured, or no forwarding classes that are being used on the interface can have rewrite rules configured. On any physical port, do not mix forwarding classes with rewrite rules and forwarding classes without rewrite rules.
Lossless Transport Support
The QFX Series supports up to two lossless forwarding classes. For lossless transport, you must enable PFC on the IEEE 802.1p code point of lossless forwarding classes. The following limitations apply to support lossless transport:
- The maximum transmission unit (MTU) of the packets cannot be larger than 2200 bytes. (The maximum size of an FCoE packet is 2180 bytes.)
- The external cable length from the QFX3500 or QFabric system Node device to other devices cannot exceed 100 meters.
- The internal cable length from the QFabric system Node device to the QFabric system Interconnect device cannot exceed 150 meters.
| Note: Junos OS Release 12.2 introduces changes to the way the QFX Series handles lossless forwarding classes (the fcoe and no-loss forwarding classes). In Junos OS Release 12.1, both explicitly configuring the fcoe and no-loss forwarding classes, and using the default configuration for these forwarding classes, resulted in the same lossless behavior for traffic mapped to those forwarding classes. However, in Junos OS Release 12.2, if you explicitly configure the fcoe or the no-loss forwarding class, that forwarding class is no longer treated as a lossless forwarding class. Traffic mapped to these forwarding classes is treated as lossy (best-effort) traffic. This is true even if the explicit configuration is exactly the same as the default configuration. If your CoS configuration from Junos OS Release 12.1 or earlier includes the explicit configuration of the fcoe or the no-loss forwarding class, then when you upgrade to Junos OS Release 12.2, those forwarding classes are not lossless. To preserve the lossless treatment of these forwarding classes, delete the explicit fcoe and no-loss forwarding class configuration before you upgrade to Junos OS Release 12.2. See Overview of CoS Changes Introduced in Junos OS Release 12.2 for detailed information about this change and how to delete an existing lossless configuration. |
