OSPF Best practices with multiples areas

[Rowlands Sharles]

Hello Support,

I have a big network connected many cities by wan links (pic attached). R1 and R2 are in the same room in the Datacenter.

R1---R2----R3---R4----R5---- and R5 connected back to R1.

Behind each router theres is a LAN with some switches and vlans.

i configured all routres wan interfaces on area 0 (in green in the scheme).

My concern is, what is the best practice; to declare each LAN behin router to area 0 or create a different area for each router behin router.

what is the best practice regarding ospf information and database?

Thanks

[MHM Cisco World · Accepted solution]

[Richard Burts]

If we knew more about this environment we could give better advice. If it is a real network deployed for multiple cities then the suggestion from MHM to have each city in a separate area is a good starting point. If this is the case I would go a bit further and suggest that the areas for each city might be configured as stub areas (or perhaps even better as totally stub areas).

But I wonder if this is a real network. I am especially puzzled by the presence of core1 and core2 and a ring of 5 switches. It suggests to me that this is some lab environment and in that case it might make sense to have everything in area 0.

[Rowlands Sharles]

Hello Richard Burts, it's a real network and working, the issue is that all nodes on all cities are in area 0 (wan and lan for each city).

for now, we have 40 routers ID in the area 0.

[Rowlands Sharles]

HI MHM, thanks for your proposition, so the best practice is to have one area by cities (maybe totally stub) and all wan routers will become ABR.

[MHM Cisco World · Accepted solution]

maybe totally stub <<- I prefer using NSSA not totally stub in case you want LSA5 (external ospf prefix) advertise between sites

MHM

[Flavio Miranda]

@Rowlands Sharles 

 Do you intent to grow up your  LANs ? If not, you may consider keep it simple and just advertise the LAN on the Area 0. Keep topology simple for me is a very good practice.

[Rowlands Sharles]

@Flavio Miranda 

Hello, yes, the lan of each city can grow up. for the moment, all LAN in each city are in the same area 0 and we have more 40 routers ID in area 0

[Joseph W. Doherty]

Answer much depends on L3 topology on each LAN.

Are there additional routers within the LANs and how complex and extensive is it?

Remember, what OSPF is sensitive to is a large number of (graphical) edges.  Link stability is an important issue too.

Consider the difference, for OSPF, dealing with a 5 node ring topology, as shown in your diagram, vs. 5 nodes in a full mesh, and the complexity impact as you add nodes maintaining either the ring topology or a full mesh.

I'm using those two topologies as extreme examples, but without knowing more details, cannot well advise because topology and stability much determines a good design.

You describe your network as being "big", could you be more specific?  Is your "big" 5 routers, or 50, 500, 5,000?

(BTW, my last pre-retirement network was an enterprise network of about 5,000 devices, most "routing" using OSPF.  So, my experience ranges from small stand alone networks, to somewhat "big" networks. Also, from LANs to International.  One design approach does not fit all, but OSPF, can pretty well handle all the forgoing if well designed for each.)

[Rowlands Sharles]

@Joseph W. Doherty 

On each LAN, we have a standard Distribution and Access switches with Vlans. 

I found this existing network, routeurs and distributions switches on all cities are in the same area 0, with sh ip ospf database, i have more 40 routers ID in area 0. This is why i'm looking for a best practice.

The network is not so big, it has about 10 routers and few L3 switches on each cities.

[Joseph W. Doherty]

Cisco's OSPF devices?  (Cisco's OSPF implementation is often much better than a generic Brand X.  Some top tier competitors come close [e.g. Brand J].)  If so, what you have now shouldn't have OSPF issues for lack of multiple areas.  You should even have headroom for some growth.

However, it wouldn't be worthless to migrate LAN site networks to be in their own OSPF area(s).

You asked whether such areas should be stubs, and that has been endorsed in multiple replies.  However, I do not see a need for it in your size network, either now, or even with some sizable growth.  Its benefits, on modern network equipment (i.e., hardware: faster processing and more memory, more link bandwidth; software: CEF, iSPF), aren't as important as they were decades ago, plus it has its own downside and much of its benefits can also be had using another OSPF feature (ABR summarization) assuming you've done hierarchical address block allocations.