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https://docs.sophos.com/nsg/sophos-firewall/20.0/Help/en-us/webhelp/onlinehelp/index.html?contextId=routing-SD-WAN-routes-behavior

SD-WAN routing behavior

You can configure SD-WAN routes with SD-WAN profiles to reroute connections dynamically when a gateway becomes unavailable or doesn't meet the SLAs any longer.

You can use SD-WAN routes to route system-generated traffic and reply packets.

Reroute connections

To implement zero-impact failover, rerouting traffic (reroute-connection) is turned on by default.

You can turn it on or off from the command-line console as follows:

Options CLI commands
Show rerouting status show routing reroute-connection
Turn on rerouting set routing reroute-connection enable
Turn off rerouting set routing reroute-connection disable

See Routing commands.

Reroute SNAT connections

By default, Sophos Firewall reroutes all connections that don't match a source NAT (SNAT) rule. You can turn on rerouting for SNAT connections using the reroute-snat-connection CLI command. Additionally, you can turn off rerouting for non-NATed connections using the reroute-connection CLI command.

However, rerouting SNAT connections only works if you apply the same source IP address to gateways or links used for rerouting connections. For example, if you use Port-A with source NAT IP address 192.0.2.1 to route the initial connection, the rerouted connection via Port-B must also use the same source NAT IP address 192.0.2.1.

Using configurations such as masqueraded (MASQ) connections or Override source translation in an SNAT rule changes the Translated Source. An SNAT rule with different translated IP addresses or IP ranges leads to communication failures between endpoint computers and servers. So, the firewall doesn't reroute SNAT connections for these configurations.

You can turn SNAT rerouting on or off from the command-line console as follows:

Options CLI commands
Show the rerouting status for SNAT connections show routing reroute-snat-connection
Turn on rerouting for SNAT connections set routing reroute-snat-connection enable
Turn off rerouting for SNAT connections set routing reroute-snat-connection disable

See Routing commands.

System-generated traffic and reply packets

You can create SD-WAN routes and specify the gateways for system-generated traffic and reply packets. On the CLI, make sure you turn on routing for both.

Reply packets

Sophos Firewall enforces symmetric routing on WAN interfaces for reply packets. These packets use the same WAN interface as the original packets.

You can configure asymmetric routing for reply packets on non-WAN interfaces. For example, you can specify an interface other than the original traffic's interface for LAN to DMZ traffic.

Restriction

SD-WAN routes don't apply to reply packets if the original traffic uses the default route (WAN link load balance). The default route applies, and reply packets exit on the same interface they enter.

You can change the setting on the CLI. See Routing commands.

System-generated traffic

Select only the destination networks and services because the incoming interface and source networks remain unknown. For example, traffic related to services used by Sophos Firewall flows through different interfaces, depending on the type of service.

You can change the setting on the CLI. See Routing commands.

Note

  • The firewall doesn't forward system-generated traffic if you select all the configured gateways as Backup on Network > WAN link manager. You must select at least one of them as Active.
  • System-generated RED traffic on UDP port 3410 is layer 2 traffic. So, SD-WAN routes don't apply to this traffic.

Route-based VPNs with SD-WAN routes

You can use SD-WAN routes with route-based IPsec VPN connections. You can select the XFRM interface's gateway in SD-WAN profiles or SD-WAN routes. See Create a route-based VPN (any to any subnets).

To compress the IPsec tunnel's traffic to increase the throughput, go to Profiles > IPsec profiles and select Pass data in compressed format for the profile you've selected in the route-based VPN configuration.