If OSPF has only a single area it can have any area number that you choose (area 0 is not required). But if OSPF has more than one area then OSPF must have area 0 to allow the other areas to communicate with each other.
I have done an experiement with OSPF where I configured an area 1 and an area 2 and configured one router with interfaces in both area 1 and in area 2. What I found was that router with interfaces in both areas knew about the subnets in both areas. But no other router in area 1 knew any subnets in area 2. And no other router in area 2 knew any subnets from area 1.
So in a general sense you can say that OSPF requires an area 0 whenever there is more than one area in OSPF.
This is an interesting question and as far as Cisco routers go, the behaviour described by Rick is perfectly correct. However, it is important to note that Cisco does not quite follow RFC2328 (the authoritative source on OSPF) as far as this behaviour goes. If OSPF has been implemented according to RFC2328, you will find that you do not need Area 0 in your case. The routers that connect the two non-backbone areas will be considered to be ABRs and will generate summary-LSAs into each area, which would mean that each area would be accessible from the other. This is because RFC2328 stipulates that any router that connects multiple areas is an ABR, regardless of whether or not it has connectivity to Area 0.
However, Cisco uses an alternative interpretation of ABR, whereby a router is only considered an ABR if it has a connection to the backbone area in addition to connections to other areas. This behaviour is described in RFC3509.
Hi everyone, I would like to thank you in advance for any help you can provide a newcomer like myself!
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