Number of routers per OSPF area

Bassam Halabi

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Oct 23, 1996, 3:00:00 AM10/23/96

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I recommend the 40, 50 and I have seen up to 120.
These are not software limitations, the numbers are
taken from networks that have shown a good level of
stability. Networks that grow beyond these limits have
the tendancy of going all the way, if 50 works, 60
is OK, 70 is no big deal ....Once you get to these
upper limits then a small hick up can cause you problems.

SAM HALABI

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> From: Aki Mizuno <miz...@netone.co.jp>
> Newsgroups: comp.dcom.sys.cisco
> Subject: Number of routers per OSPF area
> Date: Thu, 24 Oct 1996 08:45:02 +0900
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> Hello, everyone,
>
> I know cisco says that maximum numbers of router per one area is 40-50. If there are more than 50
> routers,what happen to them, my networks?
> it is true that routers needs much memory in this situation. And it depends on the numbers of neighbors.How
> many routers do you have in youe OSPF areas? Do you have any problems?
>
> regard,
>
>

Aki Mizuno

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Oct 24, 1996, 3:00:00 AM10/24/96

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Jeffrey S. Curtis

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Oct 24, 1996, 3:00:00 AM10/24/96

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[Article reformatted to 80 columns]

I run 46 routers (advertising over 200 prefixes) in one big area, and I
have no problems with wasted CPU or memory. If my network weren't so
stable, I'd probably change this. I'd probably also change it if I had
any slow links, but as is, all of the links in my OSPF cloud are 10Mbps
or above.

Route Source Networks Subnets Overhead Memory (bytes)
ospf 683 10 211 9724 34552
Intra-area: 144 Inter-area: 0 External-1: 18 External-2: 59

<router> uptime is 7 weeks, 3 days, 21 hours, 54 minutes
Routing Process "ospf 683" with ID w.x.y.z
SPF algorithm executed 2762 times

I'm not losing any sleep over 34kB of memory used on routing information,
nor am I losing any sleep over an average of 2.2 Dijkstra runs per hour.

Jeff
--
Jeffrey S. Curtis | Internetwork Manager
Argonne National Laboratory | Email: cur...@anl.gov
9700 South Cass Avenue, ECT-221 | Voice: 630/252-1789
Argonne, IL 60439 | Fax: 630/252-9689

Sam Halabi

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Oct 25, 1996, 3:00:00 AM10/25/96

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At 01:12 PM 10/25/96 -0500, John Stogoski wrote:
>How should the number of PVC's affect the size of the area? For instance,
>what if there are six area border routers and each remote site must have a
>PVC back to every ABR? We are concerned about what happens during
>reconvergence. How will it impact the CPU requirements of the remote?
>What about the high number of ABR's?
>
>Thanks,
>John
>

The number of PVC's affects the number of neighbors a router has to
deal with when forming adjacencies and flooding Link State Updates,
which in turn affects CPU.
For backup reasons your remote sites should have PVCs to a couple of
places, six is an overkill. Also this is logical redundancy if all
your PVCs are going to the same FR switch.

The nubmer of ABRs in a backbone is not an issue, the more ABRs you have
the less areas per router you will have (2-3 recommended).

I hope this helps.

SAM HALABI

>
>
>
>

Phil

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Oct 27, 1996, 2:00:00 AM10/27/96

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Aki Mizuno wrote:
>
> Hello, everyone,

>
> I know cisco says that maximum numbers of router per one area is 40-50. If there are more than 50
> routers,what happen to them, my networks?
> it is true that routers needs much memory in this situation. And it depends on the numbers of neighbors.How
> many routers do you have in youe OSPF areas? Do you have any problems?
>

> regard,

Aki,
If you summarize your routes on your ABRs (area border routers), you
should have no problems going as high as 100 routers per area. That
being said, there are another xxxxx variables which you should look at
prior to making the leap (see RFC 1583).

Cheers!
--

|
email: aa...@issc.debbs.ndhq.dnd.ca |
_____________________________________________________________________________

Howard C. Berkowitz

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Oct 28, 1996, 3:00:00 AM10/28/96

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Phil wrote:
>
> Aki Mizuno wrote:
> >
> > Hello, everyone,
> >
> > I know cisco says that maximum numbers of router per one area is 40-50. If there are more than 50
> > routers,what happen to them, my networks?
> > it is true that routers needs much memory in this situation. And it depends on the numbers of neighbors.How
> > many routers do you have in youe OSPF areas? Do you have any problems?
> >
> > regard,
>
> Aki,
> If you summarize your routes on your ABRs (area border routers), you
> should have no problems going as high as 100 routers per area. That
> being said, there are another xxxxx variables which you should look at
> prior to making the leap (see RFC 1583).
>

In general terms, the work done to compute a routing table
from link state information is:

NumberOfInterfaces * log(NumberOfRouters)

These are interfaces involved in the route calculation
process, so, considering that OSPF uses designated routers
for multiaccess media, NumberOfInterfaces is roughly the
number of media in the area, plus the number of inter-area
and external routes injected into your area.

In a specific area, OSPF workload -- and scalability -- are
highly related to:

1) The number and stability of media and routers inside
the area
2) The number and stability of inter-area routes injected
into the area
3) The number and stability of external routes injected
into the area

#1 above predicts how often the routing table needs to be
recomputed. You need to see every link inside your area, so
this isn't anything that can helped by summarization.

#2 is where summarization on the ABR helps. It helps both
in reducing the total number of routes to the number of
summaries, and in avoiding the need for recomputation
involving routes contained in summaries.

ABR summarization also helps Area 0 scalability, because it
reduces the number of routes injected there.

#3 can be helped by summarization on the ASBR and various
stub area techniques.

----

Howard Berkowitz
PSC International
(adv.) see upcoming OSPF workshop announcement at
http://www.clark.net/pub/hcb/ospfc.txt

Mic Longhurst

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Oct 31, 1996, 3:00:00 AM10/31/96

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On Thu, 24 Oct 1996 08:45:02 +0900, Aki Mizuno <miz...@netone.co.jp>
wrote:

>Hello, everyone,
>
> I know cisco says that maximum numbers of router per one area is 40-50. If there are more than 50
>routers,what happen to them, my networks?
> it is true that routers needs much memory in this situation. And it depends on the numbers of neighbors.How
>many routers do you have in youe OSPF areas? Do you have any problems?
>
>regard,

Biggest area I am running has approx 85 routers and it isn't causing
any problems.

Mic Longhurst