This article provides information on why self-generated traffic does not take the egress interface IP as its source.

By default, locally generated traffic takes the source IP address of the egress interface, which is the interface through which the route to the destination is established. However, in this particular scenario, the traffic uses the IP address of the loopback or the first configured interface as its source, rather than the egress interface.

This behavior is expected when the SRX is configured with the command 'set system default-address-selection'.

The default address selection follows this sequence:

1. The router selects the primary address from the loopback interface lo0, excluding 127.0.0.1.
2. If no suitable address is found on the loopback interface, the primary address from the primary interface is chosen.
3. When multiple interfaces have both "primary" and "preferred" addresses, the interface with the lowest index is selected, and its primary address is used. If no primary statement is explicitly set for any interface's IP addresses, the system defaults to the numerically lowest address on that interface.
4. Any remaining interfaces with IP addresses, including management or internal interfaces, may be chosen. To manage default address selection, it's advisable to assign a loopback address or explicitly configure a primary interface.

default-address-selection