What's the consecuence of applying "default-information originate" into OSPF different processes

[Lucas Reveand]

Hello, we have a topology that includes two sites, lets call them site A as the principal one that has internet connection and a site B as the branch. Both sites are connected through an MPLS network (that uses BGP) and to protect traffic we're using IPSEC VTI tunnels and running OSPF to route traffic from one site to the other.

We need internet connection in site B, so we need to propagate a default gateway from the router on site A to the router on site B. The best way I've found so far is to use the default-information originate command into the routers A OSPF process that i want to have internet access (the router that has a default gateway into his routing table).

We have three different OSPF processes into routers A and B (for others reasons)... the question is, if I apply the "default-information originate" into one process, will that change affect the other two processes?? I dont want that the other two processes change their default gateway. How does this works?

Thanks so much in advance for the help! Lucas.-

[paul driver]

Hello

It wont have any effect on any specific routes being advertised into the different ospf process however for any prefix that isnt in the rib of the spoke router traffic will be forwarded via the default route it receives from the hub router this is assuming the spokes also have multiple ospf process running.

Now if the spoke router doesn't  run the opsf process you are advertising the default in then that router wont receive the default anyway.

[Richard Burts]

There is much that we do not know about your environment and some of that might change our answer. But in general the principle is that changing something in one OSPF process will not have any impact on other OSPF processes. Each OSPF process maintains its own Link State Database with the entries that it has learned from its neighbors. OSPF processes its own LSDB and generates routes to be inserted into the local IP routing table. If you add default information originate in one process on router A then that process will advertise a default route to its neighbor process on router B. It will not have any effect on the other OSPF processes running on router B.

I am trying to think of an example that could illustrate what I am describing. Let us assume that router A has 3 OSPF processes, OSPF 10, OSPF 20, and OSPF 30. Would that imply that router B also has the 3 OSPF processes? Does each OSPF process on router B form a neighbor relationship with the corresponding process on router A? For this example let us assume that you add default information originate in OSPF 10 on router. A. Now that OSPF process will advertise a default route to its neighbor on router B. The OSPF process on router B will receive the advertised default route and create an entry in its LSDB. And then it will propose that default route to be included in the routing table of router B. Depending on how router B had learned its default route the new route from OSPF 10 might, or might not, replace the existing default route.

HTH

Rick

[Lucas Reveand]

Hello, thanks for your quick response. Let me refrase just to be sure i got it right.

The spokes do have diferent OSPF processes, lets say we have three processes numbered 10, 20 and 30. Same processes on the hub and the spokes. If I apply the default-information originate command into process 10 spokes learning from that process will learn the default Gateway... but processes 20 and 30 will not change their default Gateway. 

Am I right? or should I take a closer look. Thanks 

[paul driver]

Hello

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@Lucas Reveand wrote:

Hello, thanks for your quick response. Let me refrase just to be sure i got it right.

The spokes do have diferent OSPF processes, lets say we have three processes numbered 10, 20 and 30. Same processes on the hub and the spokes. If I apply the default-information originate command into process 10 spokes learning from that process will learn the default Gateway... but processes 20 and 30 will not change their default Gateway. 

Am I right? or should I take a closer look. Thanks 

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Unfortunately no that will NOT be the case -  the spokes Will receive the default route and as such any unknown destination WILL be forwarded via the advertised default.

However this can be negated by also applying Policy Based Routing  (PBR) to offset the default route

[Richard Burts · Accepted solution]

We still do not know enough about this environment to provide really good answers. The original poster has clarified that the hub has 3 OSPF processes and that the spokes also have 3 OSPF processes. It is still not clear whether each OSPF process on the hub establishes neighbor relationships with the corresponding process on the spoke. And it is not clear what each OSPF process advertises to its neighbors. It would be one thing if the prefixes advertised by process 10 were unique and not advertised by any other process. It is a different thing is some prefixes (including the default route) advertised by process 10 were also advertised by one or more of the other processes. 

There is another aspect of this discussion that I would like to address. The original poster says this

but processes 20 and 30 will not change their default Gateway

In a technical perspective this is absolutely true. But it does not mean what the original poster probably thinks it means. When we look at what happens when there are multiple OSPF processes, each process maintains its own Link State Data Base. And it is absolutely true that what is advertised in process 10 has no impact on what is advertised in process 20. So adding default information originate in process 10 has no impact on process 20 and would not cause process 20 to change its default. But it is almost certain that what the original poster is really concerned about is what happens in the IP routing table not what happens in the LSDB. If the hub has been operating with a default route advertised in process 20 and they add default information originate in process 10 then the process that maintains the routing table (and note that OSPF is not the process that maintains the routing table) has a choice to make. If the default newly advertised through process 10 is more attractive than the default advertised by process 20 then the IP routing table will be updated and the default route will change.

HTH

Rick

[Richard Burts]

I am glad that my explanation was helpful. It is an interesting discussion about a particular behavior of OSPF and I think other participants will find it helpful. Thank you for marking this question as solved. This will help other participants to identify discussions which have helpful information.

HTH

Rick