OSPF-area

[miracle_david@yahoo.com]

When should I need to have more than 1 area in OSPF?

Thanks

[Georg Pauwen · Accepted solution]

Hello,

the general rule of thumb is that an area should not have more than 50 routers. It all depends on your overall design, and the stability of the connections. 

Have a look at the link below:

https://www.ciscopress.com/articles/article.asp?p=1763921&seqNum=6

[Seb Rupik · Accepted solution]

Hi there,

It is not possible to filter route updates between routers in the same area, as the LSDB must be the same for all routers in the same area. However you can filter and summarise routes between areas, so by having multiple areas you limit the scope of route distribution.

By having less entries in an LSDB a router can reconverge quicker.

cheers,

Seb.

[omz]

Hi

just to add to whats been already mentioned .. 

as you may know .. a flapping link/route in an area would cause the recalculation of the spf algorithm .. with different areas .. flaps in one area would not cause the spf to re-run in another area. 

[paul driver · Accepted solution]

Hello

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miracle_david@yahoo.com wrote:

When should I need to have more than 1 area in OSPF?

Thanks

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It can shrink the Link State Database of ospf so to reduce the amount of type 1/2 LSA's that are needed to be flooded to every rtr in the same area.

Example , Say you have 8 rtrs in a single ospf area with a broadcast network, That’s 8 type 1 LSAs for each network link and at least one type 2 LSA’s (Designated router), Which will be flooded to each router in that single area now if you split these 8 rtrs into two areas then the LSDB will be reduced by half with each area receiving a type 3 LSA for the others areas type1/2 LSAs..

Multiply this with a large number of rtrs then you see how splitting a single large ospf area can be beneficial as it can reduce the amount of SPF calculation and also the cpu/memory processing of each rtr running ospf.

[Vishnu Vardhan S]

Ideally if you are going to cross more than 40 routers in OSPF area you can start using another area as the number of LSA's in area 0 will become huge. 

If you have 5 routers in one area then you are generating 8 Type 1 LSA and 4 Type 2 LSA total 12 LSA, now imagine of having 50 routers it will over burden your LSDB.

Every routers interface generates Type 1 and every DR generates one Type 2 LSA

R1----R2-------R3------R4-------R5

Can someone let me know what will happen if my LSDB is over burden.

[Joseph W. Doherty · Accepted solution]

"When should I need to have more than 1 area in OSPF?"

That's the 64 bit question.

Unfortunately, determining that answer (correctly) is almost a black art.

As Georg mentions, there was an old rule-of-thumb not to have more than 50 routers per OSPF area, but that was based on when typical OSPF routers had less powerful hardware than they do today, nor does it account for the complexity (or lack there of) of the topology, or does it account for the quality of the OSPF implementation. (Of course the flip side is many devices can have many more routed links than they had years ago too.)

The reason for having OSPF areas is to support scalability, both by containing the impact of computing the area's SPF tree and to also allow control of route prefixes entering or exiting an area.

Also understand, not all vendors OSPF implementations are equal in quality. Cisco's is one of the best. For example, to minimize the impact of re-computation of the SPF later Cisco implementations support ISPF (incremental SPF), which re-computes the SPF for the part of the topology that really needs it. Or, Cisco has back-off timers to mitigate the impact of flapping links. Or, Cisco paces out distribution of its LSAs.

The answer to your question is very much a "it depends". The "depends" rests upon many factors, one of which, like designing/using redundancy for resiliency is designing OSPF for a certain level of resiliency too. You can use additional OSPF area to mitigate possible OSPF issues impacting your whole network, but it doesn't come for "free".

In some ways, your question is much like the question of how many hosts should be allowed on any one network prefix. That too depends on many factors, although we have rules-of-thumb for that too, like a /24 is usually a good choice but like with OSPF areas, the environment might require a much smaller prefix address block or conversely allow a much larger prefix address block.