Well.. This is not a CCNA based question. It an nterview
question asked to me, though i went there on CCNA based. I m
trying to explain here...

There are few rules of router applicable only in OSPF. these
are - 

a. An internal router (IR) has all the interfaces within an
area.

b. A backbone router (BR) has at least one interface
assigned to area zero (Area o).

c. An area border router (ABR) has interfaces connected to
two or more areas.

d. An autonomous system boundary router (ASBR) has interface
in inside & outside the OSPF routing domain.

There are some rules of thumb for Cisco router OSPF designs:

A router should be in at most 3 areas (and watch your CPU 
and RAM). 
An area should contain 50 to 100 routers (I've also seen 25 
in some Cisco documents). 
A router should have fewer than 60 neighbors. 

so shain your expalnation is wrong this is an interview 
question and its comes from expernce

"A router should be in at most 3 areas"...... Cud u explain
this statement..!!!!

OSPF is a link-state routing protocol. Which is required a hierarchical network structure and the structure divided into area like backbone area, regular area, and stub area.  Every area must be connected to area 0 or backbone area. Each router can communicate with in an area and area id should be same for an area routers. If an router want to communicate with the other area router  it must be connected through  ABR router.

on the behalf of question asked by shain on my answer 

if your network has fewer than 50 to 100 routers, you may 
be able to just have one big  (happy?) area 0.

When you try a real OSPF design, you'll find that OSPF area 
0 tends to grow on you. I've been calling this the "vacuum" 
effect: OSPF area 0 almost seems to pull links into it, 
until you find your design is all one big area 0, and you 
have to start over. My advice is to keep OSPF area 0 small. 
If area 0 is small, it's easy to keep it robust. I prefer 
to put the links connecting to non-backbone areas into 
those areas, and keep them out of area 0 
                                   
jitendera kumar sinha
network and security admnistrator

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