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George90
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OSPF without area 0

Mon Mar 27, 2017 6:58 am

I'm moving my network from RIP to OSPF.

After reading documentation and examples I was confused what to put in backbone area. So I decided to skip backbone area.
Backbone area is disabled on all routers.

Now I have 8 routers in 4 areas and they exchange routes between each others without any problems. All documentation and examples (written for cisco) says that OSPF can't work without any problems. So what is wrong here ? How my network work without backbone area?
 
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Re: OSPF without area 0

Mon Mar 27, 2017 11:47 am

Not recommended to set up OSPF without backbone. All routing between areas is done over backbone area.
 
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Re: OSPF without area 0

Mon Mar 27, 2017 4:27 pm

Two questions I would have are:

1) Why do you want to avoid using the backbone area?

2) Do you plan to add more areas (or asked differently - do you plan to grow your network?)
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George90
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Re: OSPF without area 0

Mon Mar 27, 2017 5:38 pm

Crazy thing is that I don't see any route exchange problems.

@IPANetEngineer:
I don't know what to put in backbone area.

I created simple network map, take a look and give me suggestion what to put in backbone area. Color of line represents same IP and broadcast domain. So, yellow is 192.168.3.0/24 for example.
In case of expanding network, that will be most likely from location 4.
Untitled Diagram.jpg
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Re: OSPF without area 0

Mon Mar 27, 2017 5:56 pm

For a network this small, just use backbone area everywhere.

One of the basic tennets of OSPF is that there must be connectivity to the backbone area. (area 0)

It may work w/o it but that's just you getting lucky that it wants to work anyway.

If you really want areas, then I'd suggest the purple network be the backbone, but again, I don't think it's necessary for you.
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Re: OSPF without area 0

Mon Mar 27, 2017 6:01 pm

One thing we all seem to not answer is: how is it working for him?
I too agree that a backbone is required, but why is he not having problems?


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Re: OSPF without area 0

Mon Mar 27, 2017 6:41 pm

You can experiment, create separate OSPF areas, but connect them all through the backbone.
I use different subnets for the areas.
As they said above, this would clearly be overkill but it gives you some sense about how to work with OSPF using advance features.
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Re: OSPF without area 0

Mon Mar 27, 2017 7:47 pm

One thing we all seem to not answer is: how is it working for him?
I too agree that a backbone is required, but why is he not having problems?
I could be remembering wrong, but I think I've set up Cisco routers in a lab and they worked without any active area 0 interfaces in the mix, too. I think the big thing is that if you tried to add ANOTHER non-zero area, that's when you'd run into issues because OSPF must go through area zero to to reach the other area. More specifically, for the OP's situation, I think his OSPF is operating in a mode where the local area is basically acting like it's been isolated from the backbone, but still tries to keep reachability to whatever's in the local area.
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Re: OSPF without area 0

Mon Mar 27, 2017 9:09 pm

Ah! That makes sense. :)

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Re: OSPF without area 0

Mon Mar 27, 2017 11:18 pm

Requiring all non-backbone routes to go through the backbone is a loop-prevention mechanism.
Connecting non-backbone OSPF areas at an ABR

If 2 areas aren't connected through area 0 (discontiguous), how does OSPF behaving as a link state protocol increase the possibility of routing loops?

As we saw above, OSPF uses distance-vector behavior to send routes through the Area 0 backbone. Distance-vector protocols have well-known limits, such as the count-to-infinity problem. OSPF would be vulnerable to the same issues, if we didn't have boundaries on its behavior.
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George90
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Re: OSPF without area 0

Wed Mar 29, 2017 6:20 am

I wanted to update you all

Everything works as should. I didn't saw any bad effects of running OSPF without backbone.
Tomorrow I will put everything to backbone or I will use purple line as backbone, just to be sure that everything is 100% correct :)
 
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Re: OSPF without area 0

Tue Dec 18, 2018 6:49 pm

If the diagram is a single OSPF area, and that area isn't Area 0, my question would be where are the HELLO packets flowing? That would answer the question. That would determine how the LSDB is being managed. Single Area OSPF has these characteristics: 1) All routers must do algorithm, 2) Many LSAs are processed on every router, 3) Large routing tables. Looks like you have about 5-6 different routes to advertise and that may be keeping your routing table small. But if those networks have multiple sub-networks, then......IDK.....
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TheCiscoGuy
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Re: OSPF without area 0

Sun Jan 06, 2019 10:30 am

* messed up *
Last edited by TheCiscoGuy on Sun Jan 06, 2019 10:32 am, edited 1 time in total.
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Re: OSPF without area 0

Sun Jan 06, 2019 10:31 am

FYI, if all routers belong to the same area, it does not need to be area 0 (backbone). It is not recommended to use OSPF in this way mostly because you may run into scaling issues in the future. You should always ask should you not can you. I would put all routers in the backbone and be done with it, this network is small and it gives you the ability to segment the network into other areas later
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