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This article explains the Routing Change and Session Fail-over with SD-WAN

Let us consider the three Interfaces port 1, 2 and 3 are configured over an SD-WAN interface and participating in a Performance SLA.

If the Failure Threshold was crossed for port3 (5 consecutive probes remained unanswered), the SLA state of port3 switched to dead for health-check SLA_ISPs.
# FGT-1 # diag sys virtual-wan-link health-check SLA_ISPs
Health Check(SLA_ISPs):
Seq(1): state( alive ), packet-loss(0.000%) latency(50. 871), jitter(1.862) sla_map=0x0           <----- Port 1
Seq(2): state( alive ), packet-loss(0.000%) latency(80. 789), jitter(0.534) sla_map=0x0           <----- Port 2

Seq(3): state( dead ), packet-loss(5.000%) sla_marOx0                                             <----- Port 3
If update-static-route is enabled, all static routes over port3 via next-hop are removed from the routing table.
# FGT-1 # get router info routing-table database
5              *>. [1/0] via, portl
               *>                   [1/0] via, port2
                                      [1/0] via, port3 inactive
- After a routing change, sessions are re-validated and may fail over to another SD-WAN member
Example of routing change:
1) The order of oif interfaces in the policy-route changes
2) An SD-WAN member switches to dead state
3) Dynamic route update

- After removal of the static routes, existing sessions over port3 must be re-validated against the firewall policies.

The following constraints apply for re-validation of NAT sessions:

- NAT sessions are only re-checked against firewall policies if this setting is explicitly enabled:
# config system global
set snat-route-change enable
- NAT sessions are only re-checked against firewall policies performing NAT with the same “NAT-IP”
- For e.g., session fail over between Internet accesses is possible only if the same “public IP-range” is used to NAT traffic via all ISPs (BGP/dynamic routing peering needed)

Note: Here, Configuring the IP-Pool over the policy and having route via Dynamic peer is very important.