# Defining Multi-Link routes

When you use Multi-Link routing, traffic can use multiple network connections to reach its destination.

You can define Multi-Link routes for Security Engines and Virtual Engines and for both IPv4 and IPv6 traffic.

Note: Multi-Link for outbound load balancing is only supported for IPv4 traffic.

NetLink elements represent the network connections for Multi-Link. Usually, a NetLink element represents an ISP connection. However, NetLinks can also represent a leased line, xDSL, or any other type of network connection mediated by your engine.

There are two types of NetLinks: static and dynamic NetLinks.

- Static NetLinks are supported in the routing configuration for both IPv4 and IPv6 traffic.
- Dynamic NetLinks are supported only with Single Engines. Dynamic IP addresses are not supported for Engine Clusters.

Note: If you configure wireless Multi-Link on a Modem Interface of a Single Engine, only Dynamic NetLinks are supported. Modem Interfaces always have dynamic IP addresses.

A Router or a NetLink element represents a next-hop gateway that forwards packets to networks that are not directly connected to the Security Engine. Tunnel interfaces for route-based VPNs do not use Router or NetLink elements. Instead, networks that are reachable through the VPN tunnel are added directly to the tunnel interface as if they were directly connected networks.

Figure: NetLinks in the Routing tree view

This illustration shows a Multi-Link default route to the Internet using the Any network element through the ISP A and ISP B NetLinks. We recommend using separate network interfaces for each NetLink.

For each NetLink, a range of IP addresses is defined for applying NAT to the source IP address of an outbound connection that goes through the NetLink. Element-based NAT or a NAT rule in the Engine Policy defines the Outbound Multi-Link element that is used for outbound Multi-Link connections.

Defining Multi-Link routes consists of these general steps:

1. Create a NetLink for each alternative route.
2. Add Networks under the NetLinks in the Routing tree to define a route.
3. (Optional) Configure route metrics or ECMP.