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ospf network design


We are building three small data centers. BGP and OSPF will be introduced to route the traffic.

I have a question about configuring OSPF.

We don't have many routers but only two or three routers in each DC. When configuring the OSPF area on the routers,
can I put all the routes in the same area, like area 0? Or they have to be in different areas, for example, routers in DC0 with area 0, 
routes in DC1 with area 1? 

Thanks in advance for your help.

Julio Carvajal



There is no restriction about that, No worries,


When we mention the fact about using different Areas on a network is because we have several routers in the network and because of that a big routing database so if we split the network into different areas the LSA exchange and flooding of information will be less as the ABRs will advertise information on a distance-vector fashion.


Now I imagine the data centers will be connected via BGP and inside each data center OSPF will be used, if that's the case there is nothing to worry at all as each site will be on it's own AS.





Julio Carvajal
Senior Network Security and Core Specialist

Hi Jcarvaja,

Thanks for the reply. So does that mean I can either configure all the routers with area 0, or different area id for routers at different data center? 


Thanks again.



Exactly, it will not make any difference as at the other datacenter they will all look as LSA Type 5 OSPF routes. so no worries about it.


When you need to worry is when having to connect a datacenter with OSPF as the routing protocol and having an area separated by another area (The use of Virtual-links comes into place here)





By the way remember to rate all of my answers with the stars at the bottom of each post

Julio Carvajal
Senior Network Security and Core Specialist

Thanks Jcarvaja again for your help. 


I completely agree with Jcarvaja,

1)If you have less number of routes,

2)If there is less probability for the links to go down

3)If the routers are capable enough to handle the OSPF algorithms even if links go down,

then yes you can put them in area 0.

4)If you feel that there are some links which frequently go down then put those routers in non back bone area.

Try this link

5) OSPF floods all link-state changes to all routers in an area. Routers with many neighbors have the most work to do when link-state changes occur. In general, any one router should have no more than 60 neighbors.

OSPF uses a CPU-intensive algorithm. The number of calculations that must be performed given n link-state packets is proportional to n log n. As a result, the larger and more unstable the area, the greater the likelihood for performance problems associated with routing protocol recalculation. Generally, an area should have no more than 50 routers. Areas that suffer with unstable links should be smaller.

A router must run the link-state algorithm for each link-state change that occurs for every area in which the router resides. Every ABR is in at least two areas (the backbone and one adjacent area). In general, to maximize stability, one router should not be in more than three areas.

In general, the DR and backup designated router (BDR) on a multiaccess link (for example, Ethernet) have the most OSPF work to do. It is a good idea to select routers that are not already heavily loaded with CPU-intensive activities to be the DR and BDR. In addition, it is generally not a good idea to select the same router to be the DR on many multiaccess links simultaneously.




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Just wanted to mention, the number of routers per area is very much "it depends" kind of answer.  The "no more than 50" is a very, very rough (and old) rule of thumb.  Newer devices can usually support larger topologies, and if ISPF is active, even larger still.


You only need more than one area needed by scaling considerations.  Multiple areas will require an area zero.


If you only need one area, probably true in this case (six or so routers, total?), the area can be any number.  Whether to use area zero or not, now, might depend on where you might want area zero if you do need to go to multiple areas in the future.


Basically, moving to multiple area would probably require renumbering the area used by some routers.  So, you might plan for a multiple area that would require the least renumbering.

Thanks Thanveer. The info is very helpful.