Why is PFC set per priority rather than for the whole port?
Why A is the answer
Other traffic classes can keep flowing
Only the lossless class pauses.
In detail
- Only the lossless class pauses.
- Other classes keep flowing.
- Pausing a whole port would stop everything.
Use case
An illustrative scenario, not a real incident.
RoCE traffic pauses, but management traffic on the same port keeps flowing.
See it in different ways
flowchart LR A["RoCE traffic"] --> B["QoS class"] B --> C["ECN signals congestion"] C --> D["PFC prevents drops"] D --> E["Adaptive routing spreads load"]
sequenceDiagram participant S as Sender participant W as Switch participant R as Receiver S->>W: RDMA traffic W->>R: Marks packets with ECN R-->>S: Congestion notification S->>S: Slows down W-->>S: PFC pause, if a queue fills
mindmap
root(("Port traffic"))
n1["Paused"]
n2["Lossless class"]
n3["Flowing"]
n4["Other classes"]Key point: Per-priority pause protects other classes.
Why the other options are wrong
- B. Ports can only hold one VLAN at a time
- Ports can carry many VLANs.
- C. The switch has no way to measure congestion
- Switches can measure congestion.
- D. BGP sessions need each port to stay paused
- BGP does not need that.