OSPF Area

[longdv]

Hi everyone
I have a question about running multi-area OSPF.
I have a pair of C8300s, using about 80 areas, is that safe? Each area only advertises about 5 subnets to Area 0.

[longdv]

Update

I have a pair of C8300s, using about  25 areas, is that safe? Each area only advertises about 10 subnets to Area 0.

[longdv]

Update

I have a pair of C8300s, using about  25 areas, is that safe? Each area only advertises about 10 subnets to Area 0.

[M02@rt37]

Hello @longdv 

the risk in OSPF is not the number of areas but excessive LSA churn and frequent SPF recalculations, witch can stress your ABR routeur.

Saying that, 25 OSPF areas with each area advertsing only 10 prefixes into your area 0 ? it is perfectly "safe" yes.

[MHM Cisco World]

It safe for both 25 areas and 80 areas with 10 prefix for each area.

MHM

[Giuseppe Larosa]

Hello @longdv ,

the effective load depends on the type of areas involved

Each area has its own Link State Database. Your routers being connected to backbone area 0  or 0.0.0.0 will regenerate LSAs using LSA type 3 in each area they are member of if all areas are standard area.

If all areas are standard areas you have 25 LS DBs each with 250 IP prefixes in it.

If you have one backbone area and 24 totally stub area you have one 250 prefixes LS DB for area 0 and 24 LS DB with 11 prefixes : that are the 10 area local prefixes + the O IA LSA type 3 default route injected by the ABR nodes.

In OSPF design area type is very important.

Totally stub areas have the limit of not allowing redistribution from other protocols including connected and static routes.

You can also consider the use of NSSA areas where you expect you need to redistribute something into the area.

Both stub areas and NSSA areas do not receive LSA type 5 for external routes coming from backbone.

The choice of area type is per area and you can have a mix of standard , stubby and NSSA in the OSPF routing domain

Hope to help

Giuseppe

[balaji.bandi]

its good depends how your big your routing table, i always suggest to have look performance and route propagation any delays,

is this changes to be done every day basis, one time setup and no changes take place ?

still suggest deployment guide :

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/xe-16/iro-xe-16-book/iro-cfg.html

[Joseph W. Doherty]

Insufficient information to say, although as the others have already noted, likely you'll be okay.

A very old rule of thumb, I recall, was no more than about 3 areas per ABR.  You're way past that, but modern routers, and Cisco's OSPF implementation, are way past that too.

As also noted by the others, much depends on exactly what your multi-area design is, and what's actually happening in your network.

[Joseph W. Doherty]

BTW, another issue that can impact OSPF design is the available bandwidth on the links connecting OSPF routers.  Remember, LSAs need to be "sloshed" about.  With a multi-area design, you have options that might minimize the volume of LSAs both into and out of an OSPF area.  As most networks, now adays, have routed links with (often much) more than 64 Kbps, this isn't (often) the concern it was in the past.  Still, suppressing unnecessary routes, via summarization/aggregation, is usually a good thing.

Understand, when OSPF breaks due to "over loading", it also seems to come as a very unwelcome surprise.

"Pure" RFC OSPF, in my experience, isn't that difficult to overload and break.  Cisco's implementation is much more robust (while still being RFC compatible).

I mention the foregoing because if you take a no issue Cisco OSPF network, and replace it with Brand X, or possibly even add a Brand X OSPF router, you may suddenly have a problematic OSPF network due to a less than ideal OSPF network design.

Again, there's insufficient information to say what you want to do, or already doing, is "safe".  Also again, Cisco's OSPF is rather robust, so from what you've described, good chance you'll not have an OSPF issue.

[Stefan Mihajlov]

@longdv 

Yes—80 areas on a C8300 pair is generally fine, especially if each area only contributes ~5 prefixes to area 0. The real limit is LSA/neighbor scale and SPF churn, not the raw area count. Keep it stable by summarizing on the ABRs (area x range), using stub/NSSA where you can, making access links passive, and watching SPF/LSA stats and CPU. If those stay calm, your design is safe.

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Best regards,
Stefan Mihajlov

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