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Understanding Specific Route-based Routing Within the SD-WAN Overlay

CSO supports specific route-based routing (also known as longest prefix match or LPM) within the SD-WAN overlay routing domain, ensuring a more optimized route selection. The LPM feature ensures that traffic is forwarded on the specific routes learnt from remote devices (SD-WAN devices or non SD-WAN devices).

Here are a few points about how CSO supports a more optimized route selection with the help of LPM:

  • CSO installs specific routes in the SLA VRF tables (default, default-reverse, Internet, and MPLS), ensuring that the traffic is forwarded on the shortest path using specific routes.

  • CSO applies LPM to the traffic destinations that include provider hub IPVPN, enterprise hub data centers, and site-to-site traffic via enterprise hubs or provider hubs.

  • The site-to-data center traffic uses specific routes from the SLA tables. It also uses the branch site’s direct tunnel to the enterprise hub which hosts this data center. The traffic does not have to traverse an extra hop via the primary enterprise hub.

  • The site-to-IPVPN traffic uses specific routes from the SLA tables. This traffic takes the provider hub route (tunnel) to reach IPVPN. The traffic uses an enterprise hub only when the provider hub is not reachable directly.

  • As data forwarding is done using LPM, supernet and subnets can coexist within the SD-WAN department.

  • CSO applies LPM to each department in case network segmentation is enabled. LPM is applied to the default VPN in case network segmentation is disabled.

  • CSO depends on the default route only as a last resort.

As part of the LPM feature, CSO lets you configure a tenant to either route traffic through the user-selected primary and secondary hubs or prefer the shortest routes for traffic. To do this, you can use the Primary/Secondary Hub Affinity feature on the Tenant Properties tab of the Add Tenant page. See Adding a Single Tenant for details.

CSO supports two hub affinity modes - enabled and disabled. By default, hub affinity is enabled. See Table 1 to see how the hub affinity configuration influences route selection.

Table 1: Route Selection by SD-WAN CPEs Based on the Hub Affinity Mode

When Primary/Secondary Hub Affinity is Enabled

When Primary/Secondary Hub Affinity is Disabled

  • Honors the user-selected primary and secondary hubs (enterprise hubs and provider hubs).

  • Ensures that traffic is forwarded over the primary hub as long as the primary hub is active.

  • Prefers enterprise hubs over provider hubs.

  • Prefers the shortest path (the one with lesser hop count) to reach the remote destination.

  • Treats the user-selected primary and secondary hubs with the same preference.

  • Prefers the route via the enterprise hub when the cost of the routes via the enterprise hub and the provider hub is the same.

  • Prefers the enterprise hub with the lowest router-ID when the cost of routes via two enterprise hubs is the same.

  • Prefers the provider hub when the route via the enterprise hub is costlier or unavailable.

  • Prefers the provider hub with the lowest router-ID when the cost of routes via two provider hubs is the same.

  • Considers any non-parent enterprise hub (a third enterprise hub which a branch site device connects to via DVPN) for the traffic between the branch site and all other possible destinations.