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Will any issues occur if I have a topology involving 3 subnets and two OSPF areas where one subnet is in AREA 0 and the other two subnets are both in AREA 1?
For example:
[area 1, subnet 1]---[ABR #1]---[area 0, subnet 2]---[ABR #2]---[area 1, subnet 3]
Regarding the question of splitting area 1 across the backbone (area 0):
[area 1, subnet 1]---[ABR #1]---[area 0, subnet 2]---[ABR #2]---[area 1, subnet 3]
[area 1, subnet 1]---[Router #1]---[area 0, subnet 2, end device #1]---[Router #2]---[area 0, subnet 2, end device #2]---[Router #3]---[area 1, subnet 3]
Short answer: There is no problem with your proposal...
Long answer:
Even Peter's answer, which argues that reusing area numbers is bad design, offers no proof that this is a bad design; if you examine the hyperlinks he used, there is no explanation of undesirable consequences for this design. Furthermore, the argument that you might have problems connecting R1 and R3 falls short, since the R1 to R3 link could legitimately be configured in either Area 0 or Area 1, depending on what traffic you want to transit it. The difficulties he mentions are a false dilemma.
In RFC 2328, Section 3.7 OSPF explicitly allows you to use discontiguous non-backbone areas (which are called "area partitions", below):
OSPF does not actively attempt to repair area partitions. When
an area becomes partitioned, each component simply becomes a
separate area. The backbone then performs routing between the
new areas. Some destinations reachable via intra-area routing
before the partition will now require inter-area routing.
... Also, the backbone itself must not partition.
Thus whether you use the proposed discontiguous Area 1 is just a matter of taste... some people find it illogical to use the configuration in your diagram; these people might suggest that you keep OSPF area numbers together... so you'd have to change [area 1, subnet 3] on Router #3 to [area 3, subnet 3]. Other people see no problem with reusing Area 1, since OSPF area numbers are only locally significant to the router originating OSPF hellos.
Either way, we should admit that OSPF is a remarkably flexible protocol; regardless of choosing one side or another in this debate.
area 0 OSPF designs exclusively, to limit design complexity.
Mar 28, 2014 at 12:46
This will work but it would be a poor design choice (IMHO) unless you have a specific reason for doing so.
See this discussion: Duplicate OSPF area IDs
Recommended OSPF configuration best-practices (using the SSM example)
OSPF area configuration best practices
Update #1:
For the sake of completeness, I mocked up this situation using dyanmips. The platform was Cisco 3725s running IOS ver 12.4(15)T13.
R1 (lo0 1.1.1.1, f0/0 10.12.0.1) <-> R2 (lo0 2.2.2.2, f0/0 10.12.0.2, f0/1 10.23.0.2) <-> R3 (lo0 3.3.3.3, f0/0 10.23.0.3).
Loopback interfaces of R1 & R3 were placed in area 1, all other interfaces are in area 0.
Pings were then done (e.g. R3#ping ip 1.1.1.1 source 3.3.3.3) between the R1 & R3 boxes confirming connectivity.
In my opinion I would still shy away from this architecture. Mike makes perfectly valid points (and testing confirms) that this works. OSPF areas are useful tools to control route propagation and reduce the size of the router's link state database - but they also function (to me at least) to document that I intend "this" part of the network to be separate architecturally from "that" part of the network for some reason.
If in the future you decide to connect R1 to R3 and are using the same area # then you have lucked out and backed into saving yourself a lot issues with changing area numbers or having to use virtual links. So maybe this is an argument in favor of using two area IDs (0 & 1) everywhere - but then again you did not originally intend R1 and R3 to be able to communicate directly. shrug
This is a matter of style and personal preference - there appears to be no technical reason why this shouldn't work and I never intended to represent otherwise.
Update #2
Adding non-ascii art diagram of connections - drawn linearly and as a triangle. What is intended between R1 & R3? Is area1 supposed to be contiguous or separate?
Well you normally need to use virtual links to talk to area 0 via a different non area 0 area or you connect 2 parts of area 0 through a non zero area. So virtual links do not apply. You can have a non contigious area if its not area 0, but as hinted above, not really a good idea. That being said you proposed topology will work without virtual links.
