What is the difference between switching capacity and forwarding rate?

These two specs describe switch performance but measure different things.

Switching Capacity (Backplane Bandwidth)

• Total data throughput the switch fabric can handle
• Measured in Gbps
• Formula: (# ports) × (max speed) × 2 (full-duplex)

Example: 24-port gigabit switch = 24 × 1 × 2 = 48 Gbps switching capacity

Forwarding Rate (Packet Rate)

• How many packets per second the switch can process
• Measured in Mpps (Million packets per second)
• Assumes 64-byte packets (worst case)
• Formula: switching capacity ÷ (64+20) bytes ÷ 8 bits

Example: 48 Gbps ÷ 672 bits per packet = ~71.4 Mpps needed for wire-speed

Wire-Speed Non-Blocking

A switch is "wire-speed non-blocking" when it can forward at full line rate on all ports simultaneously with 64-byte packets.

Why Both Matter

• Switching capacity: Determines maximum aggregate throughput
• Forwarding rate: Determines small-packet performance (critical for real-time protocols)

Reading Datasheets

Look for these specs together:

• "Switching fabric: 48 Gbps"
• "Forwarding rate: 71.4 Mpps"
• "Line rate on all ports"

When Small Packets Matter

• IEC 61850 GOOSE (typically 50-200 bytes)
• PROFINET RT frames
• VoIP RTP packets
• Industrial control protocols

Oversubscription Warning

Some cheap switches advertise "24-port gigabit" but have oversubscribed uplinks or shared bandwidth blocks. Verify non-blocking claims for critical applications.