Merging two OSPF area 0 backbones together.

[jt.silas]

Hello everyone, I would like to know the best way to merge two OSPF area 0 backbones together. Does anyone have a link or any documentation on the best way to perform this task.

Thanks in advance.

[daniel.bowen]

How do you mean merge? Do you mean move routers from two backbone areas into the one area, or connect two backbone areas?

Daniel,

[jt.silas]

I have a project to connect two seprate companies together. Both companies are running OSPF, and they both have a area 0 backbone, how would I go about mergering the two together. I plan on creating a common area such as area 3 between both companies area 0 backbone, and using virtual links to tie both backbones together. I was looking for some documentation that goes over this, in detail.

Thanks,

[amdcent]

This is not a reply to your question but a question just came to my mind when I was reading this.

Why would some documents suggest to use Virtual link when merging two OSPF domains.

Why can't you connect Area0 to Area0 directly without going thru another area?

-amdcent

[jt.silas]

Good question. I think if you have two companies merging together, with two area-0 backbones, the extra area between them is just a demarc for each company. In-order to talk between both area-0 backbones you need virtual links to tie the areas together.

Thanks,

[svermill]

I don't think virtual links are ever recommended as a long-term solution. This is a temporary phase-in approach. You should probably be looking for a converged area 0 in the end.

Search on CCO for the OSPF Design Guide.

[amdcent]

But what is the advantage of using virtual-link as oppose to connecting both Area0 directly from initially.

-amdcent

[svermill]

I can't say for sure from personal experience. I don't think you would ever set out to create a new area just to merge two backbones. I just can't see the logic in that. I think it is useful when you have an existing non-backbone area that for whatever reason is going to be where you *begin* to merge organizations.

[ruwhite]

Virtual links are generally used for connecting an area which is physically connected through another area to area 0--I'm not even certain they can be used to connect two area 0's. It would be better to just connect them. If you're concerned about a line of demarcation and control for some time, then run bgp between them, rather than a seperate ospf area. This would give you control over what's being sent and such. Using a v-link in ospf, even if it works, won't allow you to filter routes and implement policy.

Russ

[jt.silas]

ok, I can use bgp, but what happens when the company I purchased is considered a trusted site. Now I need to remove bgp and create one ospf area 0 backbone. I thought by created a common area between each company and using VLs to connect both area 0 backbones, that this was the best way to do so.

[rng4]

Merging two OSPF networks together (be it via a virtual link or directly merging the area 0s) requires complete understanding of the two OSPF ASes. I would no recommend using virtual links as virtual links are resigned to connect an OSPF area to area 0 via another area. Ìt is not intended for tying together two area 0s. If you think in terms of LSAs. Because you are tying together two area 0s, the net effect is that all LSAs normally present in area 0 will go through the virtual link. Thus using a virtual link should create the same net effect as directly merging the area 0s, if virtual links in this capacity work at all.

If you just want to connect two comanies together and there are no redundant links between the two, I'd recommend the low tech approach of static routing. Chose one router on each side as the ASBR. Put static routes to company A on cmpany B's ASBR, and redistribute static into OSPF. Do the reverse at company A. Of course, you may use distribute-lists to control redistribution.

Just mt ¢2

[zaziz]

Hi,

What is the physical setup looks like. Does the two comapnies have physical link between

Area 0? If not then VL or GRE may work but will not be elegant. BGP is Best in such scenarios

if allowed.

Your pain could be greatly reduced a physical pipe can be conncted between area 0 routers.

Hope this helps