OSPF Area 0

[hs08]

Hi..

If we have several routers, can we configure this all routers to use ospf area 0 only?

[Giuseppe Larosa]

Hello @hs08 ,

yes you can put all routers in the same area unless they are several hundreds.

Up to 500 routers in the same area have been reported. I have seen production network with 120 routers in area 0.

The use of OSPF multi area gives more route control as at ABR level internal routes can be aggregated or filtered.

In OSPF single area no form of route filtering is actually available.

OSPF multi area provides also scalability but in many cases the first trigger is route control.

To be noted external routes LSA type 5 cannot be filtered in a granular way and to block their propagation you can use OSPF stub areas or OSPF NSSA but inside an area or you get all of them or none of them.

Hope to help

Giuseppe

[pman]

Hi,

In addition to what @Giuseppe Larosa wrote down,
This is an interesting question, I was looking for what Cisco has to say about it in terms of OSPF Design and this is what I found:

Number of Routers per Area

The maximum number of routers per area depends on several factors:

• What kind of area do you have?
• What kind of CPU power do you have in that area?
• What kind of media?
• Does OSPF run in NBMA mode?
• Is your NBMA network meshed?
• Do you have a lot of external LSAs in the network?
• Are other areas well summarized?

For this reason, it is difficult to specify a maximum number of routers per area. Consult your local sales or system engineer for specific network design help.

Number of Neighbors

The number of routers connected to the same LAN is also important. Each LAN has a DR and BDR that build adjacencies with all other routers.

The fewer neighbors that exist on the LAN, the smaller the number of adjacencies a DR or BDR have to build. That depends on how much power your router has.

You could always change the OSPF priority to select your DR. Avoid the same router as DR on more than one segment.

If DR selection is based on the highest RID, then one router could accidently become a DR over all segments it is connected to. This router requires extra effort while other routers are idle.

https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/7039-1.html#anc37

[MHM Cisco World · Accepted solution]

put all router in one area you can do that but this have limitation 

1- large database need, the router in each area must know all link of all router in same area, that huge DB.

2- loss control of prefix inside area, ospf not like eigrp which all router in eigrp can do filter/summary,
in ospf only ABR and ASBR can do such as filter/summary 

[Joseph W. Doherty]

As the other posters have already (correctly) described, yes you can.

BTW, if using just one area, it need not be area zero.

As the other posters have also touched upon, how many routers you can have in an OSPF area is an "it depends" kind of answer.  Mostly it depends on how "powerful" (vis-a-vis OSPF computations) the router is, what the topology actually is (i.e. big difference between a ring, star and full mesh topologies), how "stable" the network is (i.e. link states changing often?), and further can be impacted by OSPF feature settings (Cisco has various OSPF timers you can adjust and later Cisco OSPF variants offer iSFP).

Years ago, Cisco used to mention limiting areas to no more than 50 routers per OSPF area, but I've seen a smaller OSPF network  area go into OSPF meltdown with many less routers with a flapping link in the area (this though was on Brand X, Cisco, and some other vendors, have added various OSPF features [not in OSPF RFC, but not really in conflict with RFC either] to minimize certain OSPF situations causing an OSPF meltdown [not all vendor OSPF implementations are as "stable/solid" as Cisco's]).