OSPF Multiarea - Area not connected to backbone

[CookieV]

I have this sceneario where i have another router wich has no connection to the backbone, how could i do the connection?

[johnd2310]

Hi,

Have a look at ospf virtual links:

https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/47866-ospfdb7.html

Thanks

John

[Martin L]

you will need ospf virtual links. but I am not sure if PT supports ospf virtual links. you should do this and all advanced labibng  in GNS3.

you can attach PT file for us to check; it must be zipped first (zip format)

Regards, ML
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[CookieV]

Thanks for the help! I managed to solved it using static routing.

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I did try virtual link before and it worked, but when I reopened the file it wasn't doing anything so I was trying an alternative. Anyway, I'll try again with the virtual link but static routing is working just fine.

My doubt will now be: wich is the best thing to do between both? static or virtual link?

Sorry, this time I'll will attach my PT file with static routing. Newbie things.

[Martin L]

hmm... I don't think static routing is solution for replacing virtual links; GRE tunnels is. not sure why or how static routing works in your PT, will check your PT file later or tomorrow;

[Richard Burts]

The original poster asks a question and I would like to suggest some answers for it "My doubt will now be: wich is the best thing to do between both? static or virtual link?" I believe that the answer depends on how you define best. If static routes provide the connectivity that you need then static routes are a good solution. They are more simple than configuring virtual links, and perhaps simple is best. 

Configuring virtual links would allow you to run a dynamic routing protocol. Running a dynamic routing protocol allows you to react to changes that take place in the network and to choose an alternate path if problems occur on the primary path. If the ability to react to changes in the network is important then probably virtual links is the best. 

We do not know much about this network. From the new router is there a single path into this network? If there is no alternate path then probably static routes are good enough to be called best. If there are multiple paths from the new router into the network the virtual links are probably best. 

[CookieV]

This clears some of my doubts.
At the moment there are no plans to have multiple paths added to the new router, but then your answer makes me have one final question. If the network is planning to grow and possibly having multiple paths in new routers, is it a virtual link the best way to approach this?
So, in conclusion, static routing could be a good solution for a simple network but, later on, can cause scalability problems. And virtual links can be an easy solution and also helps with the scalability of the network. Is this correct?

[Joseph W. Doherty]

Well, whether you use virtual links or GRE tunnels, you're extending area zero, and creating additional ABRs. So, perhaps a better approach might be to physically extend area zero to at least the routers that would be the termination router for the virtual link, or GRE tunnel, traffic. I.e. those routers in areas 2 and 3.

BTW, considering the size of your topology, you could likely "safely" just use one OSPF area. Or, if you wanted to retain your current areas, since the routers in areas 2 and 3 would be ABRs, you might make those two areas fully just area zero.

[Richard Burts]

The original poster asks an interesting questions " If the network is planning to grow and possibly having multiple paths in new routers, is it a virtual link the best way to approach this?". My first response is that virtual links are rarely the best way. Virtual link is more of a work around for an awkward implementation. Additional thoughts are:

- when bring the new router into the OSPF network does it really need to be in a separate area, or could it be included in an existing area (that does have a connection to area 0?

- if the new router does need to be in a separate OSPF area then could one of the additional links be a link that connects to area 0?

[CookieV]

First answer: Yes, it needs to be in a separte area.

Second answer: I don't think so.

To give a little bit more of context, I need a total of 8 areas for 8 different departments. Area 0 being the backbone and having the technology department in area 0 too. All of the 8 areas will be distributed in two buldings:

- Building #1 for area 0, 1, 2 and 3

- Building #2 for area 4, 5, 6 and 7

Being area 1 and 4 the ones connected directly to area 0. Something like in this pic:

I'm attaching my PT file which is working with static routing at the moment. I also have a version with virtual link, but it takes to much time for the connection to work, even using FF button.

Is there a way to implement ospf with another routing protocol that could work better for this scenario?

[Joseph W. Doherty]

The fundamental problem is, your topology, as configured with multiple OSPF areas, isn't really suitable for OSFP. For example, what areas are the router to router links going to be in? If you place them (the end side of each link) into different areas, OSPF won't exchange traffic on those links! A shared link needs to be in the same area for both routers. Further, if your router has different area links, and one of those areas is not area zero, the router won't pass one area's routing information to the other area(s).

Yes, if you "contort" OSPF enough, you can make it run, but you're making a "Rube Goldberg" network. Ditto, using another routing protocol in addition to OSPF.

If you really must have OSPF areas, make all the transit routers ABRs. Their LAN facing interfaces can still be in the areas you want.

I've yellow highlighted (in the attachment) the minimum number of links I'm recommending be in area zero. All the others can be different area numbers.

PS:

The top of diagram routers' links, i.e. side and bottom, should be in the same area, and the leaf ABR's top link should be in the area being used by their neighbor's top of diagram's router's side/bottom link.

The bottom left router's left link does not need to be in area zero.

All the "side" links might be in different (and must be unique) areas.  (Again, you might want none of those to be in area zero; keeping area zero for just your ABRs [which aligns with an OSPF backbone].)  Exception is the side link on the top most ABR could be in same area as diagram's top most router, but for your design, you would likely want to keep them different.

[Deepak Kumar]

Hi,

There are multiple solutions such as:

1. Configure Virtual links.

2. Configure GRE tunnel and run OSPF over the GRE tunnel between Area 0 and other area which is not directly connected to area o.

[Georg Pauwen]

Hello,

stringing together two virtual links should work and provide backbone connectivity for all areas without using any static routes. I have attached the working config (I added loopbacks on all routers for clarity, and removed all static routes)...