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Class of Service

  • Drop congestion notification (QFX5240-64OD, QFX5240-64QD, QFX5241-64OD, and QFX5241-64QD)—QFX5240 and QFX5241 switches can function as transit switches for drop congestion notification (DCN). DCN is a congestion management technique based on packet trimming. Rather than dropping a packet when congestion occurs, the packet’s payload is trimmed, resulting in a smaller packet that is then transmitted through a high-priority queue toward its destination. Subsequent hops in the network recognize DCN-marked packets and direct them to high-priority queues. End hosts must process the trimmed DCN packets, identify the packets dropped due to congestion, and request retransmission of those lost packets. The original sender host, upon receiving the DCN feedback, immediately re-transmits the dropped packet and adjusts the flow rate.

    DCN works only for UDP unicast traffic. To enable DCN on the switch, set the UDP port number (udp-port port-number) and a strict-high priority forwarding class (forwarding-class class-name) at the [edit class-of-service drop-congestion-notification] hierarchy level. The L4 UDP port number is set to the DCN protocol number so that the switch can identify the DCN packets.

    Then set drop-congestion-notification on an ingress interface at the [edit class-of-service interface ingress-interface-name] hierarchy level so that the interface can detect the DCN-marked packets.

  • Per-interface ingress shared buffer pool allocation (QFX5130-32CD, QFX5130E-32CD, QFX5130-48C, QFX5130-48CM, QFX5220, QFX5230-64CD, QFX5240-64OD, and QFX5240-64QD)—You can tune the buffer allocated to each priority group or interface based on the dynamic threshold value (also referred to as the alpha value).

    You can set the ingress dynamic-threshold value on a global basis at the [edit class-of-service shared-buffer ingress buffer-partition (lossless, lossless-headroom, lossy)] hierarchy level. A global alpha value is useful in certain cases, but it is not effective when the switch has ports operating at different speeds.

    For switches with ports of different speeds, you can define a dynamic threshold profile with an alpha value for each priority group and then associate the profile with an ingress interface. This configuration enables interface-specific dynamic threshold values.

    To enable per-interface dynamic threshold values, define a dynamic threshold profile at the [edit class-of-service] hierarchy level. For example:

    set class-of-service dynamic-threshold-profile profile-name ingress priority-group PG-number dynamic-threshold threshold-value

    Then assign the dynamic threshold profile to an ingress interface. For example:

    set class-of-service interfaces interface-name dynamic-threshold-profile profile-name

    An interface-specific dynamic threshold value overrides a defined global dynamic threshold value or the default dynamic threshold value.