(Re)distribute subnet in OSPF without static/interface

[Eric Snijders]

I think i have a really simple question. I want to advertise a network with OSPF, but the device that "owns" this subnet doesn't have an active interface in that subnet, and also no static. I want to use the specified subnet for NAT'ing. It's a public IP subnet so if i could save 1 IP for not having an active interface that would be great.

Is there a simple way to advertise a network through OSPF without having an active interface or static route for it?

[balaji.bandi]

Hope if i understand the question correctly,

What are you trying to achieve here, if simple advertise routes in to Area ?

what happens when the client try to access that subnet ? if there is no active device in that network ?

there no use.

BB

[a.alekseev]

R1----inside -----R-NAT------outside----R2

R-NAT can advertise default route to R1

R2 can have static route to the network which R-NAT is using for NAT, and can redistribute this route to OSPF or something else.

[Eric Snijders]

Ok, let me clearify some things. Consider the following topology:

So R1 and R2 are running OSPF on the 10.0.0.x/24 subnet.

R1 uses R2 to get to the internet.
In the topology R1 is a router, but in my case this should be a Cisco ASA Firewall (so no loopback interfaces possible).

Now let's say i have the WAN subnet of 20.0.0.x/24. I want to use that subnet for NAT'ing on R1 (for example, i want to NAT PC1 to 20.0.0.1). I want to advertise this 20.0.0.x/24 subnet in OSPF so R2 knows about it, but because R1 doesn't have a interface or static route to 20.0.0.x/24, no advertisements are sent.

Since it's a ASA, i can't create a loopback interface. I could make a static route for the 20.0.0.x/24 subnet on R1, but since we're running router-on-a-stick (or just subinterfaces in case of a ASA) i have no idea where to point the static towards, since i want to be using the 20.0.0.x subnet for NAT'ing for different VLAN's (subinterfaces).

[a.alekseev]

Place a static route on R2 pointing to R1, for 20.0.0.x

[Eric Snijders]

Yes, that was my alternative. I guess there is no way to advertise this properly in OSPF without having an active interface in the 20.0.0.x subnet on R1, right?

[a.alekseev · Accepted solution]

https://www.cisco.com/c/en/us/td/docs/security/asa/asa92/configuration/general/asa-general-cli/route-static.html

On ASA You can create static pointing to null0 interface and redistribute it

[Eric Snijders]

And i can still use the subnet pointing towards the null0 interface for NAT'ing? That would be great!