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ospf area 0 only

TimeOut
Level 1
Level 1

Hi anyone 

I have some question about important normal area in ospf     Now   how do you think create normal area  that I mean  area 1 2 3 4 .... 

Because in the past the router has low performance such as cpu memory and link low speed  but now the router has high performance 

So now do you recommendation create another area yet ?    or  i just  area 0   

 

 

4 Replies 4

Martin L
VIP
VIP


how many OSPF areas or just one main area 0 will depend on how large is company, number of networks, sites, any remote sites, etc. It does not only depend on speed of routers, speed of network lines, and network performance. of course, faster routers and networks may be significant when creating areas.

 

Regards, ML
**Please Rate All Helpful Responses **

@Martin L makes good points. I would add a few things to consider.

- when a link changes state (up to down or down to up) every router within the area must perform the shortest path first calculation. If the network has only a single ospf area then every router must do the calculation for every link. If the network has multiple areas then only routers within that specific area must do the spf calculation. For some networks this is a significant consideration but for other networks not so much so.

- in some network environments routers need full information about external routes and individual routes for the entire enterprise network. But in some environments some routers may need only information about some of the networks. With multiple areas OSPF enables configuration of stub areas, totally stub areas, not so stubby areas. This may be a significant advantage for some networks and not important for some networks. 

- in some networks it is important that all routers have full information about all of the networks and subnets within the enterprise network. But in some networks some routers may need detailed information about only locally connected networks. With multiple areas OSPF supports summarization of networks at area boundaries.

HTH

Rick

Giuseppe Larosa
Hall of Fame
Hall of Fame

Hello @TimeOut ,

the main reason for using a multi area OSPF design in most cases is to achieve some level of control over route propagation.

In OSPF route filtering (of internal routes ) is possible only at area boundary on ABR nodes, for external routes is not possible to have a fine control however we can use specific type of OSPF areas like stubby or NSSA to avoid injection of external routes into a non backbone area.

In a single area domain there is no way to stop or limit the flooding of an LSA and no way to use route summarization to hide some network flap.

Hiding the internals of each area provides also increased stability, but nowdays this is more a side effect then the main goal.

 

Hope to help

Giuseppe

 

Joseph W. Doherty
Hall of Fame
Hall of Fame

BTW, when you have only a single OSPF area, it doesn't have to be area zero.  (Planing for a possible multi-area OSPF topology in the future might be better starting with using a non-zero area rather than zero.)

 

Your question is very, very difficult to answer as there are many variables that go into whether you need a multi-area OSPF network.  (As also noted by @Martin L, especially him writing "etc.")

 

The whole point of OSPF areas is to allow scalability.  At some point, too large an OSPF area will keep its routers so busy dealing with OSPF, they are unable to keep the topology "up".  In a "nothing wrong" network, with modern routers, you can often go rather "large" before hitting the breaking point.  In a "something wrong" network, like with a flapping (as mentioned by @Giuseppe Larosa), the number of router limits may come way, way down that can keep the network "up").  (BTW, years ago I did see one area in a large multi-area Enterprise OSPF network [NB: only the non-Cisco OSPF routers were impacted - vendor implementation of OSPF is one of those "etc". items.] go into OSPF meltdown due to flapping link.  Although the multi-area (also using ABR route summarization) did limit the "meltdown" to just one area, it wasn't really too helpful since that area held about 80% of the user base.)

 

More "etc." considerations:

 

@Richard Burts writes: "If the network has only a single ospf area then every router must do the calculation for every link."

 

That's true unless you're using Cisco's incremental SPF feature.  (More info: https://networklessons.com/cisco/ccie-routing-switching-written/ospf-incremental-spf)  Then it only recomputes the part of the OSPF SPF tree impacted by the change.  Of course that's both good and bad news as the impacted portion of the SPF tree might still be a large portion of the tree or it could be some "leaf".  I.e. the SPF calculation time may be much more variable.

 

@Giuseppe Larosa writes: "In OSPF route filtering (of internal routes ) is possible only at area boundary on ABR nodes, for external routes is not possible to have a fine control however we can use specific type of OSPF areas like stubby or NSSA to avoid injection of external routes into a non backbone area."

 

That's fully true on an ABR, but we can do fine control against route prefixes on a Cisco OSPF ASBR, including between different OSPF areas.  I.e. you can have a multiple OSPF areas in your topology without having any being a multi-area OSPF topology.  In the former, each OSPF area is effectively its own AS.  (I mention this because if you decide you need more than one OSPF area, this is another option rather than using an OSFP multi-area topology.)

 

Giuseppe also writes: "In a single area domain there is no way to stop or limit the flooding of an LSA and no way to use route summarization to hide some network flap."

 

I believe, again, he is 100% correct, but wanted to add there can be things done to mitigate the impact of such a situation.  Using such techniques could impact how large an OSPF area you might "safetly" use.

 

Lastly, there's an old rule-of-thumb to not exceed more than fifty routers within an OSPF area, but the value of fifty could be way, way off due to the many variables in Martin's "etc."

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