SD-WAN allows you to select different outbound WAN links based on performance SLAs. It is important that BGP neighbors are aware of these settings, and changes to them.
BGP can adapt to changes in SD-WAN link SLAs in the following ways:
- Applying different route-maps based on the SD-WAN's health checks. For example, different BGP community strings can be advertised to BGP neighbors when SLAs are not met.
- Traffic can be selectively forwarded based on the active BGP neighbor. If the SD-WAN service's role matches the active SD-WAN neighbor, the service is enabled. If there is no match, then the service is disabled.
In this topology, a branch FortiGate has two SD-WAN gateways serving as the primary and secondary gateways. The gateways reside in different datacenters, but have a full mesh network between them.
This example shows how route-maps and service rules are selected based on performance SLAs and the member that is currently active. Traffic flows through the primary gateway unless the neighbor's health check is outside of its SLA. If that happens, traffic routes to the secondary gateway.
BGP NBR1 is the primary neighbor and BGP NBR2 is the secondary neighbor.
The branch FortiGate's wan1 and wan2 interfaces are members of the SD-WAN. When the SD-WAN neighbor status is primary, it will advertise community 20:1 to BGP NBR1 and 20:5 to BGP NBR2. When the SD-WAN neighbor status is secondary, it will advertise 20:5 to BGP NBR1 and 20:2 to BGP NBR2.
Only one of the primary or secondary neighbors can be active at one time. The SD-WAN neighbor status is used to decide which neighbor is selected:
- Primary: The primary neighbor takes precedence if its SLAs are met.
- Secondary: If the primary neighbor's SLAs are not met, the secondary neighbor becomes active if its SLAs are met.
- Standalone: If neither the primary or secondary neighbor's SLAs are met, the SD-WAN neighbor status becomes standalone.