Two subnets/vlans on WAN uplink port?

[SandevSingh]

Hi Folks, I have a customer build where we need to uplink the MX 250 appliances to the Cat9k. Both sides is a trunk port. The "internet port" on the MX will be connected to the Cat9k. How can I create 2 X subnets on it so that one of them is an incoming vlan to the MX and the other outgoing vlan? Say vlan10 in the incoming vlan, so the Cat9k has a default-route to the MX on vlan10, and then the MX has a default-route on vlan20 towards the upstream ISP going via the same Internet uplink.

[Adam]

May need to do that via LAN ports instead of WAN.  Then set a default route to the vlan20 interface etc.  But maybe others would have had more experience with this.  

[jdsilva]

Because the MX NAT's all traffic moving from WAN to LAN this setup will not work. You will break state on your flows as return traffic will appear to come from a different address than what the source was sending to. 

As Adam said, you could do this on the LAN side easily enough, but then you have to worry about how does the MX talk to the cloud.

I find this ask very curious. What use case do you have where this is required? 

[PhilipDAth]

@jdsilva is right.  This wont work.