An NSX administrator wants to create a Tier-0 Gateway to support equal cost multi-path (ECMP) routing. Which failover detection protocol must be used to meet this requirement?

Equal Cost Multi-Path (ECMP) routing is achieved by configuring a Tier-0 Gateway in Active-Active mode. In this configuration, the gateway establishes multiple BGP peerings with upstream physical routers. To ensure rapid network convergence in case of a path failure, a fast detection mechanism is required. Bidirectional Forwarding Detection (BFD) is a low-overhead protocol designed for this purpose. It provides sub-second failure detection for the forwarding paths between the NSX Edge nodes and the physical routers, enabling BGP to quickly remove a failed path from its routing table and reroute traffic over the remaining active paths.

A. Host Standby Router Protocol (HSRP) is a Cisco-proprietary protocol for Active/Standby gateway redundancy and is not used in NSX-T or for ECMP.
B. Beacon Probing is a vSphere Distributed Switch feature for detecting Layer 2 network failures; it is not a Layer 3 routing failover protocol used for ECMP.
C. Virtual Router Redundancy Protocol (VRRP) is used by NSX-T Tier-0 Gateways, but specifically for Active/Standby high availability mode, not for Active/Active ECMP. ---

1. VMware NSX-T Data Center Administration Guide

"Configure BFD for BGP" section.

"BFD (Bidirectional Forwarding Detection) is a detection protocol that you can enable for routing protocols to detect failures in the path between adjacent forwarding engines. BFD provides low-overhead

short-duration detection of failures. For example

BFD can detect a link failure in less than one second. BGP must be enabled on the Tier-0 gateway." This directly links BFD to BGP on Tier-0 gateways for fast failure detection

which is critical for ECMP.

2. VMware NSX-T Reference Design Guide

Section 4.3 "Tier-0 Gateway".

This guide discusses the two high availability modes for a Tier-0 gateway: Active-Standby and Active-Active. It explicitly states

"The Active-Active state is required for ECMP." It further explains

"BFD provides sub-second failure detection for BGP

which allows for faster reconvergence than BGP’s native timers." This confirms BFD's role in ECMP designs.

3. VMware NSX-T Data Center Administration Guide

"Add a Tier-0 Gateway" section.

Under the "HA Mode" setting

the documentation specifies the options are "Active-Active" and "Active-Standby". It notes that Active-Standby mode uses VRRP for high availability

distinguishing it from the Active-Active (ECMP) mode which relies on routing protocols like BGP with BFD.