Hello there,
I see your point now.
The fxp0 traffic isolation is there as my (and perhaps yours) tests prove. The route isolation is a different matter.
To recap, you want to have a 172.26.1.20 management network and 172.26.1.20 transit network (both with same netmask) to be reachable via the same SRX and both be present in inet.0, correct?
I'm afraid this is not possible, something has to give.
I think you have several options here:
1/ make your management network different/unique - cleanest option
2/ put your 172.26.1.20 transit network into a routing-instance
3/ use different netmask on 172.26.1.20 management network and a corresponding static route with "no-install" knob. This way 172.26.1.20 static route will not be used for transit traffic because it won't be installed in FIB. But you may see a situation where a portion of your management traffic is routed via your OSPF route if it is not matched by your new static route with different netmask. This is by far the dirtiest option, I would not use it personally if I have a choice.
Rgds
Alex