Overview
Forward builds its model from the devices it collected. When a path needs to cross a segment Forward didn't collect, such as the public internet, a provider's MPLS core, or a carrier circuit, the model has nothing to forward through. The path stops at the edge device, and results show a gap.
A synthetic device fills that gap. It represents the uncollected segment as a device in the model, attached to the collected devices on either side, so paths continue across it. You configure what the segment does (which sites it connects, which subnets it carries); Forward builds a modeled device from that configuration and threads paths through it.
What a Synthetic Device Is and Isn't
- It models connectivity you can't collect. You don't own the provider equipment in an L3 VPN, the far side of the internet, or the bridge inside a carrier's L2 service, so you can't collect any of them. A synthetic device describes their effect on forwarding without collecting them.
- It is not a replacement for collection. If a device is reachable and you have credentials, collect it. A synthetic device is a coarser model: it captures the connectivity, not the full configuration and state of a real device.
- It is not live traffic or prediction. Forward evaluates a synthetic device from the snapshot, like the rest of the model. A synthetic device changes what paths exist; it doesn't send packets.
How a Synthetic Device Attaches to the Collected Network
A synthetic device is a set of connections. Each connection ties the device to a collected device's interface, the point where your network hands traffic off to the uncollected segment. On the L3 types (Internet node, Intranet node, Adjacent network, L3 VPN), a connection names:
- the collected interface where the handoff happens (the uplink), and the L3 gateway that routes to it,
- the VLAN on that link, if any,
- optionally a site the connection belongs to, and
- how the subnets reachable across the connection are determined, either entered by hand or discovered automatically from the gateway device's state.
The L2 types (L2 VPN, WAN Circuit) don't route, so their connections have no gateway, site, or subnets — just the interface, an optional VLAN, and an optional name. Configuring Connections covers each of these choices in detail. They are where most misconfiguration happens, so it's worth reading before you build a device with more than a couple of connections.
Where Synthetic Devices Appear
You create a synthetic device, but Forward assigns its location automatically. A synthetic device shows up in every location where it has a connection, derived from the collected devices it attaches to, so there's no location field to set when you create or edit one. The Sources page marks these devices as auto-assigned.
Two types of synthetic device are exceptions, which Forward anchors to a fixed location instead: the auto-generated Missing Peer, and synthetic optical devices.
Automatic placement applies to releases 25.10 and later.
The Types at a Glance
There are seven types. You add five of them (Intranet node, Adjacent network, L2 VPN, L3 VPN, WAN Circuit) from the Add synthetic device wizard; the internet node is an edit-only singleton in its own drawer, and the Encryptor is created through the REST API. Choosing a Device has the details.
| Type | Models |
|---|---|
| Internet node | The public internet or third-party public IP infrastructure. |
| Intranet node | A private inter-site network (leased L3, hub-and-spoke). |
| Adjacent network | An external network you send to and receive from but don't route through. |
| L2 VPN | An L2 broadcast domain extended over a provider WAN. |
| L3 VPN | A provider L3 VPN / MPLS core, without collecting the PE/P routers. |
| WAN Circuit | A point-to-point L2 link through a provider. |
| Encryptor | A device that encapsulates traffic in an IPSEC tunnel. |
The Missing Peer and synthetic optical devices aren't in this table because you don't create them from the type list. Forward auto-generates the Missing Peer, and it derives synthetic optical devices from collected optical equipment.
Choosing a Device explains which one to pick for a given situation and how they differ. For the full behavior and field set of each type, see Reference.