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If I have 4 routers, R1, R2, R3, and R4, and put all of the subnets from R1 and R2 in area 0, and put all of the subnets from R3 and R4 (including the routers) into OSPF area 1, would the ospf areas be shared such that OSPF on router R1 corresponds to OSPF on R2, and same with R3 and R4? And if so, how do you communicate between areas?

  • How are the routers connected? – Ron Maupin Jul 10 '16 at 21:57
  • Via serial cable...is it signficant, the ports by which they are connected? – idontevenknow Jul 10 '16 at 22:05
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    No, I mean what is the topology? Which routers are connected to which routers? You should edit your question to include this information. – Ron Maupin Jul 10 '16 at 22:05
  • OH! Sorry. Um, well I guess there are two topologies I can think of: four routers all interconnected to eachother in the shape of a box with an x...and the other one would just be r1 to r2, r2 to r3, r3 to r4 (and back to r1), like a box. – idontevenknow Jul 10 '16 at 22:07
  • Which router(s) in you topology would be member(s) of both areas (ABRs)? You should edit your question to clarify. – Ron Maupin Jul 10 '16 at 22:11
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OSPF is arranged in areas, and all inter-area traffic must pass through the backbone area (Area 0). A router with interface(s) in Area 0 and a different area is an ABR (Area Border Router), through which traffic can pass between areas. You cannot, for example, have a router with one interface in Area 1, and another interface in Area 2 and expect it to pass traffic between those two areas; all inter-area traffic must pass through Area 0. This helps to prevent routing loops.

The OSPF routers in a particular area have a full understanding of all the routes, paths, and routers in that area. An ABR, as a router in a non-backbone area and Area 0, has that knowledge of both areas.

  • So router areas are global across all connected routers: R1 has all interfaces in area 1 and R2 had all interfaces in area 1, so all the interfaces are collectively in area 1? – idontevenknow Jul 10 '16 at 22:46
  • @idontevenknow, yes. Both routers will know about all the routes, paths and routers in an area. OSPF creates and shares a database of this within an area. Each router then has all the necessary information to send traffic to the destination via the best path. – Ron Maupin Jul 10 '16 at 22:51
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I suppose a good analogy is to think of a businesses private network as an OSPF area, and then on the exterior a business will have a DMZ area which allows connections to the world-wide Internet (another area).

Thus, each of your networks can only communicate with the "outside world" from its edge (like the DMZ). That's one reason you're being asked for the topology: how specifically do the parts of your net relate to one another?

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