07-18-2025 02:57 PM
Hello.
My network is fully EIGRP at the Data Center, but there is going to be another brand integrated at the DC, so need to use OSPF on different blocks.
I would like to understand what could be the problems I could face if I use the next topology:
[OSPF Área 0] Block 1 ←→ (redistribution) ←→ [EIGRP - Switches Core] ←→ (redistribution) ←→ [OSPF Área 0] Block 2
or
[OSPF Área 1] Block 1 ←→ (redistribution) ←→ [EIGRP - Switches Core] ←→ (redistribution) ←→ [OSPF Área 2] Block 2
I understand that I may encounter routing loops if appropriate filters are not used, but something else I should take into consideration?
07-18-2025 02:59 PM
Use GRE tunnel over eigrp
Make GRE in ospf area 0
And hence you can connect split area 0 of ospf
MHM
07-18-2025 03:47 PM
Well, but there are many other blocks that are using EIGRP, and need to share routes from Block 1 and Block 2 to them. The GRE tunnel would just isolate both blocks.
Also there will be a third block using OSPF.
07-18-2025 03:51 PM
Block1 and Block2 interconnected via other link?
Ospf process number is different in each blocks?
MHM
07-18-2025 04:01 PM
Hello
Fyi- each ospf site (block) is isolated from the other ospf site (block) via eigrp
iit doesn’t matter the interconnecting network is “another routing process “
each ospf site will see the other ospf site routes as external
07-18-2025 04:58 PM
Cisco Advanced Services have said it is not recommended to have 2 or more area 0 independently. But that was another scenario / another case.
Thats why I asked if there is something I should consider.
07-18-2025 08:10 PM
@Fernando Hernández wrote:
Cisco Advanced Services have said it is not recommended to have 2 or more area 0 independently. But that was another scenario / another case.
Thats why I asked if there is something I should consider.
Possibly what Cisco Advanced Services is thinking of is "classical" partitioning of OSPF area zero or future (like suppose you wanted to convert from EIGRP to OSPF) OSPF multi-area design considerations, but as @paul driver describes, effectively, each OSPF block is its own AS. So, what you use for OSPF area numbers doesn't matter in what you're doing now.
As we have very little information about the network as now, or perhaps future, cannot suggest what you might want to consider, but basically, in large topologies, where with EIGRP you might be doing summarization, with OSPF, you need to use its multi-area capabilities for that and for other reasons.
You also mentioned there will be a 3rd block. If another "leaf", like blocks 1 and 2, it can be treated the same.
BTW, I mention the possible elimination of using EIGRP. Toward that end, you could activate OSPF on all the L3 devices, and run it in parallel with EIGRP, which may avoid the need to do any redistribution and would allow removal of EIGRP, whenever convenient. It also would avoid the need of careful mutual redistribution filtering to avoid routing loops.
07-19-2025 04:26 AM
Oh, thinking some more on your question, one point overlooked, that's been mentioned, is the devices to be added devices are Brand X. A possibly very important consideration, there's Cisco's OSPF versus Brand X's OSPF. The latter, possibly ranging to being almost as good to very inferior. That being the case, if you believe you may ever eliminate EIGRP, then possibly it may be best to assign each OSPF block a unique, non zero, area number, like in your second area number assignments proposal in OP.
Again, for what you propose doing now, and as @paul driver initially replied, area number assignments don't matter. Likewise, logically, rather than OSPF, you could use any other routing protocol, even EIGRP, while these new blocks are their own ASs, and you're doing mutual redistribution.
Lastly, depending on how large the block topologies will be, and how inferior its OSPF is compared to Cisco's, they may need to be their own multi OSPF area topologies. If so, using the suggested block area numbering is probably, for its ASBR, rather than area zero is probably, still the better approach.
The forgoing might be another reason Cisco Advanced Services recommended what they did
07-19-2025 05:13 AM
BTW, you may already have this in mind, but thought it might be worthwhile to also mention, for the mutual redistribution, your edge OSPF blocks may only need a default route to EIGRP and EIGRP might take a single, or few, summary prefixes from each OSPF block.
07-19-2025 08:32 AM
The advice about "it is not recommended to have 2 or more area 0 independently" was based on the assumption that it was a single OSPF network. What the OP is describing is 2 independent OSPF networks with EIGRP in between. Each OSPF network would see subnets of the other OSPF but would see them as EIGRP external routes and not as OSPF routes.
07-19-2025 09:59 AM
@Richard Burts wrote:
The advice about "it is not recommended to have 2 or more area 0 independently" was based on the assumption that it was a single OSPF network. What the OP is describing is 2 independent OSPF networks with EIGRP in between. Each OSPF network would see subnets of the other OSPF but would see them as EIGRP external routes and not as OSPF routes.
Rick, unsure whether you appending your reply to one of mine (i.e. "in response to Joseph W. Doherty") was just to add it as just another reply (which I suspect is the case) or whether you believe something I wrote is contrary to what you just wrote.
In case the latter, I fully agree with what you wrote, i.e. proposed topology is two (or three, based on a later reply) independent OSPF networks, with EIGRP in between. (In fact, in one of my replies, I mentioned you could use any other routing protocol for the "blocks", even EIGRP, and make them independent networks too. I.e. OSPF topology issues, in what's proposed, would be limited to each OSPF [independent] AS.)
As to Cisco Advanced Services recommending not to use more than one area zero, it doesn't apply to the proposed topology, as initially noted by Paul. So was Cisco Advanced Services "blind" to that, or might they have deeper reasons? And if deeper reasons, are there other possible OSPF considerations too? I suspect understanding potential OSPF design issues, is what OP is seeking.
Hopefully, my prior replies touched upon what other OSPF design issues might be contingent upon. Unfortunately, without more information of what the proposed network will immediately encompass or may encompass in the future, cannot be more precise in what might be the best OSPF design approach now.
To recap, with the information given, edge "block" OSPF area numbering don't immediately matter.
In the future, if you need a multi area OSPF topology, area design and configurations, including numbering, can be a very big deal. As area zero is the exclusive backbone area of a multi OSPF area topology, how to use it is of great concern. Also, as partitioned OSPF areas is NOT good, especially for area zero, using a unique non zero area number, for each OSPF edge block, is likely the best approach.
Such as:
[OSPF Área 1] Block 1 ←→ (redistribution) ←→ [EIGRP - Switches Core] ←→ (redistribution) ←→ [OSPF Área 2] Block 2
07-19-2025 10:31 AM
Joseph
I was not responding particularly to your post, but was just adding at the most recent post. And I do NOT disagree with anything that you have said.
And a further comment to the OP: you suggest 2 scenarios, one has OSPF area 0 on both sides, the other has OSPF area 0 and OSPF area 2. Either one would work. In general I believe that the first scenario (two area 0) is more common and the second (area 0 and area 2) more unusual. But it depends on the topology of the networks. If the right hand network has several OSPF areas, and if there is an OSPF area 0 that is different from the OSPF area connecting to EIGRP, then using area 2 is quite appropriate.
07-19-2025 11:26 AM
@Richard Burts wrote:
I was not responding particularly to your post, but was just adding at the most recent post. And I do NOT disagree with anything that you have said.
BTW, commentary on my posts, even/especially disagreements, always welcome (even when I'm, embarrassingly, mistaken). One nice thing about these forums, it's sort of "peer reviewed".
@Richard Burts wrote:
And a further comment to the OP: you suggest 2 scenarios, one has OSPF area 0 on both sides, the other has OSPF area 0 and OSPF area 2.
Did I miss that? (I may have.)
OP has both "blocks" using areas 0 and 0 or areas 1 and 2. Both are considered possibly problematic by Cisco Advanced Services, implying using areas 0 and a non-zero?
As Rick describes, using area zeros on all edge blocks, should work fine, and is likely what most would do. Also, as Rick describes, either of the two OP examples should work.
Rick does also describe the right side block having an OSPF area topology, but as long as the two blocks stay as independent ASs, each OSPF block doesn't any same AS OSPF design consideration relative to the other block OSPF AS. I.e. again, area numbering doesn't matter.
If, at some point, you do join these separate OSPF ASs into one OSPF AS, then area numbering, and area types, really matter. Which is why, in the very first reply, MHM's, he specifies placing a GRE tunnel into area zero, although he didn't mention what the rest of the OSPF area allocations should be. The latter would be an "it depends", but whether you use a tunnel (or tunnels), or run OSPF across your Cisco devices, again working with Brand X, good chance you'll would want the blocks to be in their own OSPF areas, which is why, I suggest, assigning each block a unique, non-zero, OSPF area number. Again, doesn't matter if blocks remain as their own ASs, but may be what you would want to do with the whole network running OSPF.
07-19-2025 11:52 AM
Joseph
I agree that we are fortunate to have what is essentially a peer reviewed environment. We all have occasionally said something that was not quite on the mark (and sometimes just wrong) and are fortunate to have colleagues that will point that out.
07-19-2025 12:47 PM
Hello
as i’ve mentioned already and confirmed by @Richard Burts @Joseph W. Doherty - you will be okay to use ospf at both sites in the same area if need be when you have “another” routing process like eigrp in between those sites (blocks)
Discover and save your favorite ideas. Come back to expert answers, step-by-step guides, recent topics, and more.
New here? Get started with these tips. How to use Community New member guide