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Synopsis: Network A needs to be able to use a static default route to Network B via a local connection (G0/0/3). Network A does NOT want to route ANY Network B addresses through the rest of its network (currently a number of subnets are being received by other routers inside Network A).

Option 1: Use separate OSPF processes. on Network A's router form an OSPF adjacency with Network B using OSPF process 1. And then form all other OSPF adjacencies within Network A using OSPF process 2.

Option 2: Set the default route on the Network A router to Network B on G0/0/3, then put static routes for the rest of Network A via G0/0/0. This could be a minimal list of subnets since Network A is in big blocks typically. CON: requires human intervention and coordination.

Option 3: Use OSPF inbound filtering using route maps with a distribute list. We can use a route map to prevent OSPF routes from being added to the routing table by matching on the routes coming into gi0/0/3. What I don't know about this is if they'll still be advertised to other routers within Network A. Would the distribute list prevent the redistribution of Network B's routes?

Caveat: Network A admins have no control over Network B's routers. This is why all above options include solutions applied to Network A's router only.

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  • You should edit your question to include a diagram of what you mean. For example, a single router could have both network A and network B attached, and there would be no need for a routing protocol, like OSPF. OSPF also has stub areas, and you could also inject a default route into OSPF and use things like distribute lists.
    – Ron Maupin ♦
    Aug 21 '18 at 15:25
  • "Caveat: Network A admins have no control over Network B's routers. This is why all above options include solutions applied to Network A's router only." Then you would not use OSPF between separate ASes. You would use something like BGP that is designed for separate ASes.
    – Ron Maupin ♦
    Aug 21 '18 at 15:26
  • @RonMaupin You're right, I need to provide a diagram to better reflect my question. Is it possible to link to a graphic outside of NE without getting in trouble? I'm realizing that my question doesn't reflect my issue very well. I'll likely recreate the question in a clearer fashion separate from this one.
    – dutsnekcirf
    Aug 21 '18 at 16:53
  • You can embed an image in the question.
    – Ron Maupin ♦
    Aug 21 '18 at 16:55
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As @ronmaupin points out, this is a scenario that OSPF was not designed for and works very poorly at. OSPF was designed assuming you have full control over the network, and the only determination of the path from A to B is the cost. Every router has a full view of the network so that it can make the best path decision.

BGP, on the other hand, was designed for exactly this type of scenario, where the routing decisions are based more on policy than a simple metric. BGP assumes you don't have control over the entire network, so your routers can decide (based on policy) what routes to advertise and what routes to accept.

There are kludges to make OSPF work in this case, but they are kludges, band-aids, and tricks that will fail when you need it the most, or will confuse an engineer months from now when they (even you) try to troubleshoot it.

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  • Marking this as the answer as I believe it does indeed cover the basic gist of the question. It might be worthwhile to at least detail what one of the kludges may look like in order to show that it is indeed possible to filter or prevent redistribution of routes in an OSPF environment.
    – dutsnekcirf
    Aug 21 '18 at 16:56

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