Understanding Default CoS Settings

Default Forwarding Classes and Queue Mapping

Table 1 shows the default mapping of the default forwarding classes to queues and packet drop attribute.

Default Forwarding Class Sets (Priority Groups)

If you do not explicitly configure forwarding class sets, the system automatically creates a default forwarding class set that contains all of the forwarding classes on the switch. The system assigns 100 percent of the port output bandwidth to the default forwarding class set.

Ingress traffic is classified based on the default classifier settings. The forwarding classes (queues) in the default forwarding class set receive bandwidth based on the default scheduler settings. Forwarding classes that are not part of the default scheduler receive no bandwidth.

The default forwarding class set is transparent. It does not appear in the configuration and is used for Data Center Bridging Capability Exchange (DCBX) protocol advertisement.

Default Code-Point Aliases

Table 2 shows the default mapping of code-point aliases to IEEE code points.

Table 3 shows the default mapping of code-point aliases to DSCP and DSCP IPv6 code points.

Default Classifiers

The switch applies default unicast IEEE 802.1, unicast DSCP, and multidestination classifiers to each interface that does not have explicitly configured classifiers. If you explicitly configure one type of classifier but not other types of classifiers, the system uses only the configured classifier and does not use default classifiers for other types of traffic.

There are two different default unicast IEEE 802.1 classifiers, a trusted classifier for ports that are in trunk mode or tagged-access mode, and an untrusted classifier for ports that are in access mode. Table 4 shows the default mapping of IEEE 802.1 code-point values to forwarding classes and loss priorities for ports in trunk mode or tagged-access mode.

Table 5 shows the default mapping of IEEE 802.1p code-point values to forwarding classes and loss priorities for ports in access mode (all incoming traffic is mapped to best-effort forwarding classes).

Table 6 shows the default mapping of IEEE 802.1 code-point values to multidestination (multicast, broadcast, and destination lookup fail traffic) forwarding classes and loss priorities.

Table 7 shows the default mapping of DSCP code-point values to forwarding classes and loss priorities for DSCP IP and DCSP IPv6.

On QFX10000 switches, Table 8 shows the default mapping of MPLS EXP code-point values to forwarding classes and loss priorities.

Default Rewrite Rules

There are no default rewrite rules. If you do not explicitly configure rewrite rules, the switch does not reclassify egress traffic.

Default Drop Profile

Table 9 shows the default drop profile configuration.

Default Schedulers

Table 10 shows the default scheduler configuration.

By default, only the five default schedulers shown in Table 10, excluding the mcast scheduler on QFX10000 switches, have traffic mapped to them. Only the queues associated with the default schedulers, and forwarding classes on QFX10000 switches, receive default bandwidth, based on the default scheduler transmit rate. (You can configure schedulers and forwarding classes to allocate bandwidth to other queues or to change the default bandwidth of a default queue.) In addition, other than on QFX5200, QFX5210, and QFX10000 switches, multidestination queue 11 receives enough bandwidth from the default multidestination scheduler to handle CPU-generated multidestination traffic. If a forwarding class does not transport traffic, the bandwidth allocated to that forwarding class is available to other forwarding classes.

Default hierarchical scheduling, known as enhanced transmission selection (ETS, defined in IEEE 802.1Qaz), divides the total port bandwidth between two groups of traffic: unicast traffic and multidestination traffic. By default, unicast traffic consists of queue 0 (best-effort forwarding class), queue 3 (fcoe forwarding class), queue 4 (no-loss forwarding class), and queue 7 (network-control forwarding class). Unicast traffic receives and shares a total of 80 percent of the port bandwidth. By default, multidestination traffic (mcast queue 8) receives a total of 20 percent of the port bandwidth. So on a 10-Gigabit port, default scheduling provides unicast traffic 8-Gbps of bandwidth and multidestination traffic 2-Gbps of bandwidth.

On QFX10000 switches, default scheduling is port scheduling. Default hierarchical scheduling, known as ETS, allocates the total port bandwidth to the four default forwarding classes served by the four default schedulers, as defined by the four default schedulers. The result is the same as direct port scheduling. Configuring hierarchical port scheduling, however, enables you to group forwarding classes that carry similar types of traffic into forwarding class sets (also called priority groups),and to assign port bandwidth to each forwarding class set. The port bandwidth assigned to the forwarding class set is then assigned to the forwarding classes within the forwarding class set. This hierarchy enables you to control port bandwidth allocation with greater granularity, and enables hierarchical sharing of extra bandwidth to better utilize link bandwidth.

Default scheduling for all switches uses weighted round-robin (WRR) scheduling. Each queue receives a portion (weight) of the total available interface bandwidth. The scheduling weight is based on the transmit rate of the default scheduler for that queue. For example, queue 7 receives a default scheduling weight of 5 percent, 15 percent on QFX10000 switches, of the available bandwidth, and queue 4 receives a default scheduling weight of 35 percent of the available bandwidth. Queues are mapped to forwarding classes (for example, queue 7 is mapped to the network-control forwarding class and queue 4 is mapped to the no-loss forwarding class), so forwarding classes receive the default bandwidth for the queues to which they are mapped. Unused bandwidth is shared with other default queues.

If you want non-default (unconfigured) queues to forward traffic, you should explicitly map traffic to those queues (configure the forwarding classes and queue mapping) and create schedulers to allocate bandwidth to those queues. For example, except on QFX5200, QFX5210, and QFX10000 switches, by default, queues 1, 2, 5, and 6 are unconfigured, and multidestination queues 9, 10, and 11 are unconfigured. Unconfigured queues have a default scheduling weight of 1 so that they can receive a small amount of bandwidth in case they need to forward traffic. (However, queue 11 can use more of the default multidestination scheduler bandwidth if necessary to handle CPU-generated multidestination traffic.)

If you map traffic to an unconfigured queue, the queue receives only the amount of group bandwidth proportional to its default weight (1). The actual amount of bandwidth an unconfigured queue receives depends on how much bandwidth the other queues in the group are using.

On QFX 10000 switches, if you map traffic to an unconfigured queue and do not schedule port resources for the queue (configure a scheduler, map it to the forwarding class that is mapped to the queue, and apply the scheduler mapping to the port), the queue receives only the amount of excess bandwidth proportional to its default weight (1). The actual amount of bandwidth an unconfigured queue gets depends on how much bandwidth the other queues on the port are using.

If the other queues use less than their allocated amount of bandwidth, the unconfigured queues can share the unused bandwidth. Configured queues have higher priority for bandwidth than unconfigured queues, so if a configured queue needs more bandwidth, then less bandwidth is available for unconfigured queues. Unconfigured queues always receive a minimum amount of bandwidth based on their scheduling weight (1). If you map traffic to an unconfigured queue, to allocate bandwidth to that queue, configure a scheduler for the forwarding class that is mapped to the queue and apply it to the port.

Default Scheduler Maps

Table 11 shows the default mapping of forwarding classes to schedulers.

Default Shared Buffer Configuration

Table Table 12 and Table 13 show the default shared buffer allocations: