An administrator is attempting to troubleshoot why the vSAN witness node cannot form a stretched cluster with the vSAN data nodes. The administrator can successfully ping the vSAN data node from the vSAN witness using the following command: vmkping -I -s -d What could be the possible cause of the issue?

The vmkping command with the specified parameters confirms that basic Layer 3 network connectivity exists between the vSAN witness and data nodes, and that the network path supports a standard MTU of 1500. However, this command only uses the ICMP protocol and does not verify if the specific TCP/UDP ports required for vSAN cluster communication are open. vSAN uses port 12321 (UDP) for its Clustering Monitoring, Membership, and Directory Service (CMMDS). If a firewall or network access control list (ACL) is blocking this port, the nodes cannot exchange membership information, which is essential for forming a cluster.

B. Port 443 is not opened bidirectionally between all nodes. This port is for vCenter Server communication. While necessary for management, it is not used for the direct internode communication required to form the vSAN cluster itself.
C. The customer does not have any virtual machines in the vSAN Cluster. The presence of virtual machines is not a prerequisite for a vSAN cluster to form. The cluster must be healthy and established before VMs can be provisioned on it.
D. Jumbo Frames have not been enabled on the Witness Network. The successful ping used a packet size of 1472, which is for a standard MTU of 1500. While Jumbo Frames are a performance best practice, they are not a requirement for cluster formation.

1. VMware vSAN Stretched Cluster Guide (vSphere 8.0), Page 17, Section: "Network Requirements for Stretched Clusters": This guide explicitly states the network requirements between data sites and the witness site. It details that while ICMP (ping) is used for basic connectivity checks, the actual vSAN traffic requires specific ports to be open.

2. VMware Ports and Protocols Tool (ports.vmware.com): This official tool lists the required ports for vSAN. For communication between ESXi hosts in a vSAN cluster, it specifies UDP port 12321 for "vSAN CMMDS" and TCP port 2233 for "vSAN RDT". A failure in CMMDS communication directly prevents cluster formation.

3. VMware vSAN Network Design Guide, Page 11, Section: "Jumbo Frames": This document clarifies that "vSAN does not require jumbo frames to be enabled... If jumbo frames cannot be enabled across the entire vSAN network, then the default MTU of 1500 bytes should be used." This confirms that the lack of Jumbo Frames does not prevent cluster operation.