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CoS on Virtual Chassis Switch Ports

Virtual Chassis devices have Virtual Chassis ports (VCPs) to interconnect members of the Virtual Chassis. VCPs are not used for external access.

Class of service (CoS) on Virtual Chassis access ports is the same as CoS on these devices when they are in standalone mode.

This topic describes CoS support on Virtual Chassis access interfaces and on VCPs.

Access Interface CoS Support

CoS on Virtual Chassis access interfaces is the same as CoS on standalone device and Node device access interfaces, except for shared buffer settings.

Similarities in CoS Support on Virtual Chassis Access Interfaces Compared to Standalone Device Access Interfaces

Virtual Chassis access interfaces support the following CoS features in the same way as access interfaces on standalone devices:

  • Forwarding classes—The default forwarding classes, queue mapping, and packet drop attributes (Table 1) are the same:

    Table 1: Default Forwarding Class Configuration

    Default Forwarding Class

    Default Queue Mapping

    Default Packet Drop Attribute

    best-effort (be)

    0

    drop

    fcoe

    3

    no-loss

    no-loss

    4

    no-loss

    network-control (nc)

    7

    drop

    mcast

    8

    drop

  • Packet classification—Classifier default settings and configuration are the same. Support for behavior aggregate, multifield, multidestination, and fixed classifiers is the same.

  • Enhanced transmission selection (ETS)—This data center bridging (DCB) feature that supports hierarchical scheduling has the same defaults and user configuration, including forwarding class set (priority group) and traffic control profile configuration.

  • Priority-based flow control (PFC)—This DCB feature that supports lossless transport has the same defaults and user configuration, including support for six lossless priorities (forwarding classes).

  • Ethernet PAUSE—This feature has the same defaults and configuration.

  • Queue scheduling—This feature has the same defaults, configuration, and scheduler-to-forwarding-class mapping. Queue scheduling is a subset of hierarchical scheduling.

  • Priority group (forwarding class set) scheduling—This feature has the same defaults and configuration. Priority group scheduling is a subset of hierarchical scheduling.

  • WRED profiles—This feature has the same defaults and configuration.

  • Code-point aliases—This feature has the same defaults and configuration.

  • Rewrite rules—This feature has the same defaults and configuration (no default rewrite rules applied to egress traffic).

  • Host outbound traffic—This feature has the same defaults and configuration.

Differences in CoS Support on Virtual Chassis Access Interfaces Compared to Standalone Device Access Interfaces

The default shared buffer settings and the way in which you configure shared buffers are the same on Virtual Chassis access interfaces as on standalone devices. The difference is that on Virtual Chassis access interfaces, the shared buffer configuration is global and applies to all access ports on all members of the Virtual Chassis, while on standalone devices, you can configure different buffer settings on different access interfaces.

You cannot configure different shared buffer settings for different Virtual Chassis members. All members of a Virtual Chassis use the same shared buffer configuration.

CPU-Generated Host Outbound Traffic

CPU-generated host outbound traffic is forwarded on the network-control forwarding class, which is mapped to queue 7. If you use the default scheduler, the network-control queue receives a guaranteed minimum bandwidth (transmit rate) of 5 percent of port bandwidth. The guaranteed minimum bandwidth is more than sufficient to ensure lossless transport of host outbound traffic.

However, if you configure and apply a scheduler instead of using the default scheduler, you must ensure that the network-control forwarding class (or whatever forwarding class you configure for host outbound traffic) receives sufficient guaranteed bandwidth to prevent packet loss.

Tip:

If you configure a scheduler instead of using the default scheduler, we recommend that you configure the network-control queue (or the queue you configure for host outbound traffic if it is not the network-control queue) as a strict-high priority queue. Strict-high priority queues receive the bandwidth required to transmit their entire queues before other queues are served. To limit the amount of bandwidth a strict-high priority queue can consume (and to prevent the strict-high priority queue from starving other queues), apply a shaping rate to the strict-high priority traffic in the scheduler configuration.

As with all strict-high priority traffic, if you configure the network-control queue (or any other queue) as a strict-high priority queue, you must also create a separate forwarding class set (priority group) that contains only strict-high priority traffic, and apply the strict-high priority forwarding class set and its traffic control profile (hierarchical scheduler) to the VCP interfaces.

Related Documentation