Create Outbound Multi-Link elements

Outbound Multi-Link elements group together NetLinks as a single entity and set options for load balancing.

You can create multiple Outbound Multi-Link elements, and each NetLink can belong to more than one Outbound Multi-Link element at the same time.

To create the Outbound Multi-Link element, you must define the following:
  • Which NetLinks are included
  • The load-balancing method for determining which link is selected for each new outbound connection
  • Whether each NetLink in the Outbound Multi-Link element is an Active or Standby NetLink
NetLink selection for load balancing can be based on either of two methods:
  • Round Trip Time — The firewall periodically probes the NetLinks to test them for speed and selects the fastest available active NetLink for each new outbound connection. NetLink performance is measured for each new TCP connection by sending the initial SYN request to the destination through all available NetLinks. When the destination host sends the SYN-ACK reply, the NetLink that receives the reply first is used to establish the TCP connection. The firewall cancels the slower connection attempts by sending a TCP Reset (RST) to the destination through the other NetLinks.

    The fastest route is automatically selected for each connection. Information about the performance of each NetLink is cached, so no new measurement is made if a new connection is opened to the same destination within a short time period.

  • Ratio — Traffic is distributed between all available active NetLinks according to the relative bandwidth of each NetLink. The NetLink with the highest bandwidth is assigned the largest portion of the traffic. The bandwidths of the other NetLinks are automatically compared to the bandwidth of the NetLink with the most bandwidth to produce a ratio for distributing the traffic.

    When the volume of traffic is low, the ratio of actual traffic distribution is approximate. When the volume of traffic is high, the ratio of traffic handled by each NetLink is closer to the ratio calculated from the link capacity.

You can optionally assign QoS Classes to NetLinks in the Outbound Multi-Link element to specify which traffic is routed through which NetLink. NAT rules can alternatively be used to select a particular link, but if you use QoS Classes, traffic can still fail over to other links if the selected link fails.

The same QoS class can be assigned to more than one NetLink in the same Outbound Multi-Link element to balance traffic through those selected NetLinks when those links are available. If you want to use QoS class to specify which traffic uses which NetLink, you must assign the QoS class to the traffic in an Access rule or with the QoS policies based on the DSCP codes in the traffic.

The Outbound Multi-Link elements you create do not work on their own; you must use them in the Firewall Policy’s NAT rules to select traffic for outbound load balancing.

Note: If you use element-based NAT and multiple external IP addresses, the default NAT address works like an Outbound Multi-Link element.

  For more details about the product and how to configure features, click Help or press F1.

Steps

  1. Select Configuration, then browse to SD-WAN.
  2. Browse to Traffic Handlers.
  3. Right-click Traffic Handlers, then select New > Outbound Multi-Link.
  4. In the Name field, enter a unique name.
  5. From the Method drop-down list, select the method for link selection.

Outbound Multi-Link Properties dialog box

Use this dialog box to define outbound multi-link properties.

Option Definition
Name Specifies the Outbound Multi-Link name.
Category Categories allow you to flexibly filter your Management Client view.
Select Opens the Category Selection dialog box.
Comment An optional comment for your own reference.
Method Defines the method for link selection.
  • Round Trip Time — The firewall periodically probes the NetLinks to test them for speed and selects the fastest available active NetLink for each new outbound connection.
  • Ratio — Traffic is distributed between all the available active NetLinks according to the relative bandwidth of each NetLink. The NetLink with the highest bandwidth is assigned the largest portion of the traffic.
Option Definition
Multi-Link Members section
NetLink Shows the selected Netlink.
Type Shows whether the Netlink is active or standby.
  • Active — Traffic is routed through the NetLink according to the Method you specify in the Outbound Multi-Link element properties.
  • Standby — Traffic is only routed through the NetLink if all primary (active) NetLinks are unavailable.
Selected Range Specifies the IP address range for dynamic source address translation (NAT) for the internal source IP addresses on this NetLink.
QoS Classes Specifies the QoS Classes for traffic handled by the NetLink.
Add Opens the Multi-Link Member dialog box.
Edit Opens the Multi-Link Member dialog box for the selected Multi-Link Member.
Remove Removes the selected Multi-Link Members.
Option Definition
QoS related settings section
Name Shows the QoS Class and the Netlink name.
NetLink Role Shows whether the NetLink is Active or Standy.
Method Shows the selected method for link selection.