Traceflow injects packets into a vSphere distributed switch (VDS) port and provides various observation points along the packet’s path as it traverses physical and logical entities (such as ESXi hosts, logical switches, and logical routers) in the overlay and underlay networks. This allows you to identify the path (or paths) a packet takes to reach its destination or, conversely, where a packet is dropped along the way. Each entity reports the packet handling on input and output, so you can determine whether issues occur when receiving a packet or when forwarding the packet.

Keep in mind that traceflow is not the same as a ping request/response that goes from guest-VM stack to guest-VM stack. What traceflow does is observe a marked packet as it traverses the overlay network. Each packet is monitored as it crosses the overlay network until it reaches and is deliverable to the destination guest VM. However, the injected traceflow packet is never actually delivered to the destination guest VM. This means that a traceflow can be successful even when the guest VM is powered down.

Traceflow supports the following traffic types:

  • Layer 2 unicast

  • Layer 3 unicast

  • Layer 2 broadcast

  • Layer 2 multicast

You can construct packets with custom header fields and packet sizes. The source for the traceflow is always a virtual machine virtual NIC (vNIC). The destination endpoint can be any device in the NSX overlay or in the underlay. However, you cannot select a destination that is north of an NSX edge services gateway (ESG). The destination must be on the same subnet or must be reachable through NSX distributed logical routers.

The traceflow operation is considered Layer 2 if the source and destination vNICs are in the same Layer 2 domain. In NSX, this means that they are on the same VXLAN network identifier (VNI or segment ID). This happens, for example, when two VMs are attached to the same logical switch.

If NSX bridging is configured, unknown Layer 2 packets are always be sent to the bridge. Typically, the bridge forwards these packets to a VLAN and reports the traceflow packet as delivered. A packet reported as delivered does not necessarily mean that the trace packet was delivered to the specified destination.

For Layer 3 traceflow unicast traffic, the two end points are on different logical switches and have different VNIs, connected to a distributed logical router (DLR).

For multicast traffic, the source is a VM vNIC, and the destination is a multicast group address.

Traceflow observations may include observations of broadcasted traceflow packets. The ESXi host broadcasts a traceflow packet if it does not know the destination host's MAC address. For broadcast traffic, the source is a VM vNIC. The Layer 2 destination MAC address for broadcast traffic is FF:FF:FF:FF:FF:FF. To create a valid packet for firewall inspection, the broadcast traceflow operation requires a subnet prefix length. The subnet mask enables NSX to calculate an IP network address for the packet.


Depending on the number of logical ports in your deployment, multicast and broadcast traceflow operations might generate high traffic volume.

There are two ways to use traceflow: through the API and through the GUI. The API is the same API that the GUI uses, except the API allows you to specify the exact settings within the packet, while the GUI has more limited settings.

The GUI allows you to set the following values:

  • Protocol---TCP, UDP, ICMP.

  • Time-to-live (TTL). The default is 64 hops.

  • TCP and UDP source and destination port numbers. The default values are 0.

  • TCP flags.

  • ICMP ID and sequence number. Both are 0 by default.

  • An expiry timeout, in milliseconds (ms), for the traceflow operation. The default is 10,000 ms.

  • Ethernet frame size. The default is 128 bytes per frame. The maximum frame size is 1000 bytes per frame.

  • Payload encoding. The default is Base64.

  • Payload value.