> ## Documentation Index > Fetch the complete documentation index at: https://docs.scalacomputing.com/llms.txt > Use this file to discover all available pages before exploring further. # Scala Switch > The Scala Switch models a shared-buffer data center Ethernet switch with PFC, ECN, ECMP hashing, and flowlet load balancing for Clos fabrics. The Scala Switch models a shared-buffer data center Ethernet switch with PFC, ECN, ECMP hashing, and flowlet load balancing for Clos fabrics. Model version 3.7.0 is the version these pages describe. ## What this model simulates The Scala Switch is a generic shared-memory Ethernet switch with IEEE 802.3 interfaces. A simulation configuration can place it as a Rack (ToR), Fabric, or Spine switch in a Clos topology. Rather than modeling a specific vendor ASIC, it models the mechanisms those ASICs share: a single shared packet buffer, per-traffic-class flow control, congestion marking, and multi-path forwarding. The results you reason about with it are buffer occupancy, PFC pause and resume behavior, ECN marking, packet drops, and how traffic spreads across equal-cost paths. Simulations on the platform carry IPv4 traffic. The platform counts a switch's bandwidth as the sum of its port speeds, each full-duplex port once, assuming a non-blocking switch. `SwitchingCapacity` is compared with that sum to show whether the switch is oversubscribed; it does not limit what the switch forwards. ## Features * **Priority Flow Control (PFC), IEEE 802.1Qbb.** Per-traffic-class pause and resume. Each port pauses its upstream sender when a lossless arrival would take its own occupancy in a buffer pool past the Xoff threshold, and absorbs in-flight traffic in per-port headroom. It resumes the sender when a lossless packet that arrived on that port leaves the switch while the port's headroom is empty and its occupancy is at or below the Xon threshold. A pause frame requests 65535 quanta, and the switch re-sends it every half pause time while the port stays paused. * **Explicit Congestion Notification (ECN), RFC 3168.** RED-style marking: packets carrying ECT(0) or ECT(1) are marked CE with a probability that rises between two thresholds on an exponentially weighted moving average of egress queue occupancy. * **Shared buffer management.** One switch-wide buffer divided into as many as eight pools, with eight traffic classes mapped onto them, separate ingress and egress accounting, and a choice of dynamic or fixed egress queue limits for lossy classes. * **Lossless and lossy traffic classes.** A class with PFC enabled pauses its upstream sender when the port's occupancy in the class's pool passes Xoff, and is never dropped at egress. [Sizing a lossless class](https://docs.scalacomputing.com/v0.1.2376/models/scala-switch/shared-buffer.md#sizing-a-lossless-class) gives the conditions under which it is not dropped at ingress either. A class without PFC is bounded by egress queue limits and pool capacity and is dropped when it exceeds them. * **Traffic classes from DSCP, RFC 2474.** Eight classes, numbered 0 to 7, taken from the packet's DSCP field divided by 8. * **Bandwidth-delay product (BDP) queue sizing.** Egress queue limits can be set as a multiple of the fabric's bandwidth-delay product. * **Equal-cost multi-path (ECMP) forwarding.** Flow-based path selection over equal-cost routes, with six hash methods. The default, `crc32WithSalt`, salts the hash per switch, which de-correlates the choices of adjacent tiers in a fabric. * **Dynamic load balancing (flowlet switching).** With `LoadBalancingMethod` set to `flowlet`, the switch moves a flow to the member of its equal-cost set with the least egress occupancy when the flow sends nothing for `Gap` or longer, and by default prefers members that PFC is not pausing. * **Statistics.** Per-port packet and byte counters, PFC and ECN counters, aggregate buffer occupancy per pool, and a load balancing record for each member of each equal-cost set. [Records a simulation writes](https://docs.scalacomputing.com/v0.1.2376/models/scala-switch/packet-handling.md#records-a-simulation-writes) lists them. ## Architecture The switch is assembled from a few cooperating components. The switch node hands every admission decision to the shared buffer manager, which owns the buffer and drives the ports. ```mermaid theme={null} flowchart TB SW["Switch node: ECMP flow hash and load balancing decision"] LB["Load balancer: member selection, flow table, load balancing record"] SBM["Shared buffer manager: pools, headroom, PFC, byte accounting, admission"] ECN["ECN marker"] BST["Buffer statistics: aggregate occupancy record"] P0["Port 0: eight traffic-class queues and a PFC queue"] PN["Port N: eight traffic-class queues and a PFC queue"] L0["Link to a peer: propagation delay"] LN["Link to a peer: propagation delay"] SW -->|"ingress and egress admission"| SBM SW -->|"decision for each packet"| LB SBM -->|"egress admission observed"| LB SBM -->|"queue averages and marking"| ECN SBM -->|"occupancy samples"| BST SBM -->|"receive and transmit"| P0 SBM -->|"receive and transmit"| PN P0 --- L0 PN --- LN ``` | Component | Responsibility | Configured by | | - | - | - | | Switch node | Computes the ECMP flow hash for each packet, asks the load balancer which equal-cost member to use, and hands every ingress and egress admission decision to the shared buffer manager. | [Switch](https://docs.scalacomputing.com/v0.1.2376/models/scala-switch/configuration.md#switch) | | Load balancer | Selects the member of an equal-cost set. With `none` it leaves the member as the flow hash modulo the set size and records what was assigned to each member; with `flowlet` it keeps a flow table and moves flows at gaps. | [Switch](https://docs.scalacomputing.com/v0.1.2376/models/scala-switch/configuration.md#switch), [Load balancing flowlet](https://docs.scalacomputing.com/v0.1.2376/models/scala-switch/configuration.md#load-balancing-flowlet) | | Shared buffer manager | Owns the switch-wide packet buffer. Divides it into pools, reserves headroom, accounts for every byte entering and leaving, sends and honors PFC frames, and admits or drops every packet. | [Shared buffer manager](https://docs.scalacomputing.com/v0.1.2376/models/scala-switch/configuration.md#shared-buffer-manager) | | ECN marker | Keeps a moving average of each egress queue's occupancy and marks ECN-capable packets as they start transmitting. | [ECN handler](https://docs.scalacomputing.com/v0.1.2376/models/scala-switch/configuration.md#ecn-handler) | | Buffer statistics | Samples ingress, egress, and shared-buffer occupancy per pool and writes the aggregate buffer record. | `BufferStatsReportInterval` in [Shared buffer manager](https://docs.scalacomputing.com/v0.1.2376/models/scala-switch/configuration.md#shared-buffer-manager) | | Port | One per connected port. Receives frames, hands them to the switch for admission, and transmits with a strict-priority scheduler over eight per-class queues plus a dedicated PFC control queue. | [Network interfaces](https://docs.scalacomputing.com/v0.1.2376/models/scala-switch/configuration.md#network-interfaces) | | Link | Carries frames between the port and its peer with the configured propagation delay. | [Network interfaces](https://docs.scalacomputing.com/v0.1.2376/models/scala-switch/configuration.md#network-interfaces) | ## How to read these pages Read them in this order: 1. [Packet handling](https://docs.scalacomputing.com/v0.1.2376/models/scala-switch/packet-handling.md): how a packet is classified, admitted, queued, scheduled, ECN-marked, paused by PFC, and steered across equal-cost paths, and what a simulation records. 2. [Shared buffer manager](https://docs.scalacomputing.com/v0.1.2376/models/scala-switch/shared-buffer.md): how the buffer is divided into pools and limits at startup, how every byte is accounted for at run time, and how to size a lossless class, with a worked example of the defaults. 3. [Configuration](https://docs.scalacomputing.com/v0.1.2376/models/scala-switch/configuration.md): every attribute a simulation configuration can set, with its type and default, how to change attributes through the platform API, and the configuration rules that stop a simulation. This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.