You are asked to set up an IP fabric that supports Al or ML workloads. You have chosen to use lossless Ethernet in this scenario, which statement is…

In a lossless Ethernet fabric designed for AI/ML workloads using RoCEv2, Explicit Congestion Notification (ECN) is a critical congestion management tool. When a switch's output queue begins to fill and reaches a configured threshold, the switch marks the ECN field in the IP header of traversing packets. The destination host receives these marked packets and sends a Congestion Notification Packet (CNP) back to the original source device. This CNP instructs the source to reduce its transmission rate, thus preventing packet loss and maintaining a lossless environment. Therefore, the switch initiates a notification process that ultimately informs the source device of congestion.

B. Only the source and destination devices need ECN enabled.

This is incorrect. All intermediate switches in the data path must also be configured for ECN to mark packets when they experience congestion.

C. ECN marks packets based on WRED settings.

While ECN thresholds are configured within a WRED profile on Junos devices, this describes the implementation detail, not the fundamental action. The primary statement of function is that the switch notifies the source.

D. ECN is negotiated only among the switches that make up the IP fabric for each queue.

This is incorrect. ECN is an end-to-end mechanism that requires the active participation of the end hosts (source and destination) as well as the network switches.

1. Juniper Networks Documentation, Understanding CoS-Based Forwarding for RoCE. "When a queue on an output port starts to fill, ECN marks a packet to notify the receiving endpoint of the congestion. The receiving endpoint (the destination server) sends a congestion notification packet (CNP) to the transmitting endpoint (the source server) to signal congestion."

2. Juniper Networks Documentation, Example: Configuring a Lossless RoCE Fabric Using ECN and PFC. This configuration guide shows ECN being enabled on the switches, which is the component that detects congestion and marks packets, thereby initiating the notification chain back to the source.

3. IETF RFC 3168, The Addition of Explicit Congestion Notification (ECN) to IP, Section 5. "An ECN-capable router detects impending congestion and marks the packet... The receiver's IP layer informs the upper-layer protocol... The transport protocol at the receiver then sets a flag in its transport-layer header to inform the sender of the congestion."