When a tier-0 logical router is running in Active-Active ECMP mode, you cannot configure stateful NAT where asymmetrical paths might cause issues. For Active-Active ECMP 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 port of the packets from 172.16.10.10 to 188.8.131.52. Having a public source address enables destinations outside of the private network to route back to the original source.
However, when there are two Active-Active tier-0 routers involved, as shown here, reflexive NAT must be configured.