Port Numbering in SONiC

When using SONiC, you may notice that port names or numbers are not continuous.

The non-continuous port numbering in SONiC is due to lane grouping and hardware architecture constraints in high-speed network switches. Here's a detailed explanation:

1. Physical Lane Grouping

High-Speed Interfaces Require Multiple Lanes

Modern high-speed interfaces (e.g., 100G, 400G) use multiple physical lanes to achieve higher bandwidth. For example:

• A 100G QSFP28 interface uses 4 lanes (each lane runs at ~25Gbps).
• A 400G QSFP-DD interface uses 8 lanes (each lane runs at ~50Gbps).

Each interface consumes a block of lanes. If the switch ASIC allocates lanes in fixed increments (e.g., blocks of 4 or 8), the port numbers will be assigned by that increment. For example:

• Ethernet0 uses lanes 0-3 → Next available lanes start at 4.
• Ethernet4 uses lanes 4-7 → Next available lanes start at 8.

2. ASIC Architecture Constraints

Fixed Lane Groupings

Switch ASICs often have predefined lane groupings for different interface types:

• Broadcom Tomahawk3: Groups lanes in blocks of 4 for 100G ports.
• Broadcom Tomahawk4: Groups lanes in blocks of 8 for 400G ports.

Example:
If an ASIC dedicates lanes 0-127 to user ports and each 100G port requires 4 lanes, the port numbers will increase by 4:
Ethernet0, Ethernet4, Ethernet8, ..., Ethernet124.

3. Split Ports and Subinterfaces

Flexibility for Lower-Speed Modes

Switches can split high-speed ports into multiple lower-speed ports. For example:

• A 100G port (Ethernet0) can be split into:
  • 4x25G ports: Ethernet0_0, Ethernet0_1, Ethernet0_2, Ethernet0_3.
  • 2x50G ports: Ethernet0_0, Ethernet0_1.

4. Platform-Specific Conventions

Legacy Compatibility

Older switches or platforms may have used different numbering schemes. SONiC maintains compatibility by:

• Following the hardware vendor's port numbering (e.g., Cisco-style Ethernet1/1 vs. SONiC's Ethernet0).