An administrator is troubleshooting an issue where workloads connected to a Tier-1 Gateway named T1-App...

Comprehensive and Detailed 250 to 350 words of Explanation From VMware Cloud Foundation (VCF) documents:

In theNSXmulti-tier routing architecture used by VCF, aTier-1 Gatewayis composed of two primary components: theDistributed Router (DR)and theService Router (SR). The DR runs as a kernel module on every ESXi host in the transport zone, facilitating East-West traffic. The SR resides on the NSX Edge nodes and provides centralized services like North-South connectivity and stateful services.

Communication between the DR (on the ESXi host) and the SR (on the Edge node) occurs over a hidden internal segment known as theRouter Link. This link is encapsulated inGenevejust like VM-to-VM traffic. When a VM attempts to reach an external destination, the packet is first routed by the DR on the local host. The DR then encapsulates the packet and sends it across the overlay to the TEP (Tunnel Endpoint) of the Edge node hosting the SR.

If theMTU (Maximum Transmission Unit)is misconfigured on the physical network or the virtual switches, large encapsulated packets will be dropped. However, small packets (like pings between VMs on the same host) might still succeed. In this scenario, the fact that the VM can ping the local DR butcannot reach the SR—and therefore cannot reach external networks—points to a failure in the transport between the host and the Edge.

If the Geneve-encapsulated packet containing the ping request to the SR's internal interface exceeds the physical network's MTU, it will fail. Since VCF 5.x/9.0 requires a minimum MTU of1600(ideally9000) for the overlay to account for the Geneve overhead, a mismatch anywhere in the fabric will break the DR-to-SR "backplane" communication. This prevents the Tier-1 from passing any traffic to its Tier-0 uplink, effectively isolating the workloads from North-South traffic.