External PHY 100G Port Breakout
Overview
The AS9947-36XKB (J2C+) platform supports External PHY 100G QSFP28 port breakout into 4×10G or 4×25G logical interfaces.
This feature enhances port flexibility, density, and migration support between 10G/25G/100G network environments.
Feature Characteristics
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Based on PAM4 SerDes MAC connected to an external 400G Gearbox PHY. |
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MAC Side: 2 × 50G PAM4 lanes |
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PHY Line Side: 4 × 25G NRZ lanes |
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Gearbox resources are shared between adjacent 100G ports. |
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Paired-port behavior: Breakout on an even-numbered port automatically disables its adjacent odd-numbered port. |
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Example: ce4 breakout disables ce5; ce2 breakout disables ce3. |
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Each 400G PHY device (PHY group) hosts four 100G ports and operates at a shared VCO rate. |
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Speed consistency is required within the same PHY group: |
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Mixed 10G and 25G breakout modes are not supported in the same group. |
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40G mode restricts breakout-capable ports in the group to 4×10G only. |
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4×25G breakout cannot coexist with 40G mode in the same group. |
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Only one breakout mode (logical or external) can be active at a time. |
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FEC is not supported in 25G breakout mode. It must be disabled on the peer interface. |
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Hotplug support: Module type must match the configured breakout mode. |
Hardware Constraints
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Each breakout consumes all four NRZ lanes of the PHY line side. |
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Adjacent port must remain disabled when breakout is active. |
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PHY resources are shared across 100G ports, enforcing mode uniformity within a PHY group. |
Benefits
Utilizing this port breakout offers several advantages:
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Provides greater flexibility for port speed and configuration. |
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Increases port density by enabling multiple logical interfaces per 100G port. |
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Simplifies migration from 10G/25G infrastructure to 100G backbone. |
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Reduces downtime through runtime mode switching (no reboot required). |
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Optimizes resource utilization via PHY pairing logic. |
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Supports existing optics inventory, including DACs, AOCs, and SFPs. |
Limitations
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Breakout is supported only on even-numbered ports. |
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Paired ports cannot be active during breakout operation. |
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Mixed breakout speeds (10G and 25G) are not supported within the same PHY group. |
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FEC must be disabled when using 25G breakout mode. |
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Correct module type must be inserted for the configured mode to function properly. |
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FEC is not functional in 25G mode. The BCM driver returns a failure during FEC negotiation; therefore, FEC must be disabled on the peer interface when operating at 25G. |
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Port breakout on CE0 is not supported. |