Best-route mode for SD-WAN

The rule #2 guarantees that, by default, the SD-WAN rules will not select a member that does not have a valid route to the destination. However, as we have explicitly highlighted, by default, SD-WAN rules look for any valid route through the member in question, even if that route is not the best route that exists in the routing table.

To be even more precise, the SD-WAN rules look for the best available route via the member in question (even if it is not the best available route globally).

For example, imagine that the SD-WAN receives a session destined to 10.4.1.1. The matched SD-WAN rule must select between the members (in the order of preference): H1_MPLS, H2_MPLS, and H3_MPLS. Consider the following routing table snippet:

S*  0.0.0.0/0  [1/0] via 192.2.0.2, port1, [1/1]
               [1/0] via H1_MPLS tunnel 172.16.1.5, [10/1]
               [1/0] via H2_MPLS tunnel 172.16.2.5, [10/1]
               [1/0] via H3_MPLS tunnel 172.16.3.5, [10/1]

B  10.4.1.0/24 [200/0] via 10.200.1.2 (recursive via H3_MPLS tunnel 172.16.3.5)     

Clearly, the best route to our destination is the route towards 10.4.1.0/24 through H3_MPLS. But will the SD-WAN select this member?

According to the rule #2, the answer is not by default. Indeed, the SD-WAN rule will check the members in the order of preference, and for each member, it will look for the best route to the destination through that member.

What is the best route towards 10.4.1.1 through H1_MPLS? Does it exist? Yes, it's the default route!
It is a perfectly valid route, therefore the H1_MPLS overlay will be selected, providing that it does not violate the configured SLA target.

If this is not the desired behavior, we can instruct the SD-WAN rule to consider only the routes that are the best routes globally, by configuring the advanced option tie-break fib-best-match:

Adding this option to the rule in the above example, will guarantee the selection of H3_MPLS.