[Guide] Multi area OSPF without area 0 router

[frknl]

Here is a OSPF topology without a individual router that specifically runs a area 0. I couldn't find an example like this with Google search, so hopefully it works for someone that searchs kinda design.

Best regards.

[MHM Cisco World]

Ospf need area 0 to connect other different area.

But using loopback and config it as area 0 work and you can connect different area.

The rfc statement that the abr must have link to area 0 to generate lsa3  and in your case you do that by loopback interface.

[frknl]

You are correct but while learning, resources never mentions this trick. The good thing, there is no need extra one router to complete area 0 link availability.

[MHM Cisco World]

I read it in some CCIE article BUT 
using it without area 0 make suboptimal and some time routing LOOP in your network.
better to use area 0.

[Joseph W. Doherty]

". . . using it without area 0 make suboptimal and some time routing LOOP in your network."

Unsure (always?) about lack of area 0 on R2, being "suboptimal" and/or creating any ("some time" - transient?) "LOOP".

Of course, such a design would be "unusual".

What should happen on R2, if it didn't have an area 0 interface, R2 would "know" the whole topology of both areas 1 and 2.  What R2 will not do, though, is "share" area 1 routes with its area 2 router neighbors or share area 2 routes with its area 1 router neighbors.

Basically, it will work much like if you had two OSPF processes or two different routing protocols (e.g. OSPF and EIGRP) on R2, without any redistribution.  NB: this is also somewhat also like a "ships-in-the-night" topology, but cannot be extended across all routers, unless you did something like use subinterfaces on the physical interfaces.

Without a full "ships-in-the-night" design, generally R1 and R3 wouldn't "know" about R4, and the converse, i.e. R4 wouldn't "know" about R1 and R3.  However, if R2 pushed a default into both areas 1 and 2 (unsure that can be done except on an ABR), then the whole network should function without area 0.

[MHM Cisco World]

see lab below

[Joseph W. Doherty]

". . . while learning, resources never mentions this trick."

Hmm, not really  a "trick".

Consider you had another router, R5, connected to R2, and the R2<>R5 link was in area 0 and R5 had other "backside" interfaces, in another area, area 3.

Areas 1 and 2 would intercommunicate via R2, just as they do now.

The key point to understand, by R2 having any interface in area 0, even a loop back, it makes the router an ABR, which will share routes (restricted, though, by area type) and route between all its connected areas.

[MHM Cisco World]

one case that Loopback help in design,
in R3 even if R5-R2 link from R4 is less than from R1 cost <R1-R2 cost 1000>
the R3 select R1,

SO I add loopback to R4 and now R3 select short cost path.

[MHM Cisco World]

as I mention before add more one ABR and even with Loopback area 0 still R1 dont know anything from R3,R4
so I give you case LO work and case that you need area 0 that connect all different area.

[Martin L]

Following CCIE RS 5.0 OCG could help

OSPF design calls for grouping links into contiguous areas. Routers that connect to links in different areas are Area Border Routers (ABR) . ABRs must connect to area 0, the backbone area , and to one or more other areas as well. It is noteworthy to mention
that RFC 2328 defines an ABR simply as a router “attached to multiple areas.” While it does not explicitly state that one of these areas must be the backbone area 0, it nonetheless implicitly assumes it throughout its contents. This slight ambiguity has led different vendors to implement ABR functionality in slightly different ways. It is therefore strongly recommended to become familiar with RFC 3509, “Alternative Implementations of OSPF Area Border Routers,” which explains in detail the Cisco approach to implementing ABR functionality. The key takeaway is that in the Cisco implementation, only a router that is actively attached to multiple areas (that is, has at least one active interface in these areas), including the backbone area , considers itself an ABR and performs the appropriate functions. A router actively attached to multiple areas but not to the backbone area does not
consider itself an ABR and does not act like one.

source: CCIE Routing and Switching v5.0 Official Cert Guide, Volume 1, Narbik Kocharians, Peter Palúch, 2015 Cisco press,

[Joseph W. Doherty]

BTW, the information Martin provided, I believe, doesn't conflict which my earlier posts (NB: not saying his posting was intended too).

Interesting, though, how Cisco has a "take" on the "ambiguity" on the meaning of an ABR.

The "ambiguity" might be due to, again, that any router with multiple area connections, which will route between those areas, might be considered an ABR, but only an ABR with an area 0 connection will process route sharing and control between areas.