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  • 1.  QFX5K Packet Buffer Architecture [Tech post]

    Posted 07-19-2023 16:41
    Edited by spuluka 10-10-2023 05:39

    Overview

    On QFX5K platforms all packets which enters ingress pipeline will be stored at central MMU packet buffers before it egress. Packet buffers are required for burst absorption, packet replication and for multiple other use cases. Without packet buffers bursty traffic will get tail dropped and leads to poor network performance.  Excess burst might be received on egress port under below scenarios.

    ·      Multiple ingress ports sending traffic to single egress port.

    ·      Traffic from high speed ingress ports to low speed egress ports.

    Packet buffers also essential to implement various congestion management applications like Priority flow control (PFC), Explicit congestion notification (ECN) & WRED.

    QFX5K data-centre switches has shallow on-chip buffer. These switches use Broadcom's Trident and Tomahawk series ASIC. This shallow on-chip buffer provides low latency packet forwarding. Managing this limited packet buffer among different congested queues and providing fair buffer access is very important. This document explains QFX5K buffer architecture in detail.

    Packet Buffers

    On QFX5K platforms, on the chip buffers are measured/allocated in the units of cells.

    Cell size varies in different platforms (refer below table). Packets are stored in multiple cells based on its size. First cell will store 64 bytes of packet meta-data (except QFX5220 where meta-data is only 40bytes) and remining space used to store actual packet. Subsequent cells only used to store packet data.

    Platforms

    Cell size (bytes)

    QFX5100, QFX5110, QFX5200, QFX5210

    208

    QFX5120

    256

    QFX5220, QFX5130

    254

     

    Packets from ingress port placed in MMU buffer before it is scheduled by egress port. With no congestion on egress queues, these packets will be dequeued from MMU as soon as they arrive. At max ~3KB buffer utilization will be seen per port based on the port load.   In case o