When a tier-0 or tier-1 logical router is running in active-active mode, you cannot configure stateful NAT where asymmetrical paths might cause issues. For active-active routers, you can use reflexive NAT (sometimes called stateless NAT).
In this example, as packets are received from the web VM, the Tenant2NAT tier-1 router changes the source IP address of the packets from 172.16.10.10 to 184.108.40.206. Having a public source IP address enables destinations outside of the private network to route back to the original source.
When there are two active-active tier-0 routers involved, as shown here, reflexive NAT must be configured.