OSPF Area

[hs08]

HI...

We have 2 router connected using OSFP, and on 1st router my OSPF config looks like this

interface GigabitEthernet0/0/0

ip address 10.102.103.3 255.255.255.255
no ip redirects
no ip proxy-arp
ip ospf 100 area 0.0.0.102

I just want to know, OSPF area in 2nd router should using same are or can be different?

If this router connect to another router, OSPF area also should using same area or can be different (so 1st router have different area for some interfaces)

[MHM Cisco World]

Same area must be use

Otherwise the ospf failed. 

[hs08]

hi @MHM Cisco World 

so each peer interface should use same ospf area, am i right?

If we have 3 router lets say A, B and C. Router A connect to B using ospf area 100 and Router A connect to C using ospf area 200. This is possible, so router A have 2 ospf area?

[MHM Cisco World]

please see comment above 
thanks 

[balaji.bandi]

OSPF - you should have Area 0 all time when you design OSPF

you can have any area, as long as all peers interface should be in same area where the interface connected.

area design best practice :

https://community.cisco.com/t5/networking-knowledge-base/ospf-design-best-practices/ta-p/3112070

[MHM Cisco World]

please see comment above 

[Spooster IT Services]

Hi hs08 ,

- The best practice is to always have area 0 configured in your OSPF topology .

- For two OPSF routers to form neighborship some parameters are exchanged in the ospf packets , and in those the " AREA id " is exchanged , and it has to be same for two routers two form neighborship. 

- Now taking the three router example :

a------b-------c.  now let's assume router a---b are in area 0 and b---c are in area 1 , in this scenario the router b which is part of two OSPF area's will start acting as the ABR ( area border router , a router with  one of it's interface in area =0  and other interfaces in other areas ) . ABR will act as the gateway , and will help in inter area routing ( sharing prefixes between different area ) 

[Joseph W. Doherty · Accepted solution]

For OSPF routers to form an OSPF adjacency, area parameters need to match, this includes area number (as correctly answered by the other posters).

An OSPF router can have different interfaces in different areas (BTW, also the "norm" for ABRs; as also correctly answered by the other posters).

What might be a bit unclear is what happens when you have a router in multiple areas without one of the areas being area zero.

E.g. R1 (area 10) R2 (area 20) R3

R2 will "know" of both areas 10 and 20 topologies, i.e. it can send packets, correctly to any place in areas 10 or 20.

However R1 will not "know" about area 20, at all, and ditto for R3, which will not "know", at all, about area 10.  I.e. either R1 or R3 cannot (knowingly) send packets to the other area.  (I write "knowingly" as there is a way, in this simple example for areas 10 and 20 to pass traffic back and forth, without area zero, but it's not a best or even good practice.  [For those wondering how to do without an area zero - hint: ASBR.])

The "correct" way for areas 10 and 20 to "know" of each other (routes), would be to make R2 an ABR, which could be done, as simply by, defining a loopback interface in area zero.

In OSPF area zero is "special" in that it allows different areas, including area zero, to exchange routes.