Hello Chidambaram,
because its routing table will be built using only LSA-3 from the backbone, ignoring LSA-3 received from other areas
Does it mean it considers only the LSA-3 originated from a valid ABR
No, that is not a correct description. Remember that LSAs are exchanged only between routers that have formed an adjacency. An adjacency always belongs to a single area only. Therefore, LSAs received over such adjacency are placed into the LSDB for the same area the adjacency belongs into. When we say that an ABR processes only backbone summary-LSAs, we are saying that the router will process only LSA-3 received from adjacencies in Area 0.
In your particular network, R1, R2 and R3 are ABRs, and R4 is an internal router in the Area 0. Adjacencies in Area 0 are therefore only: R1/R4, R2/R4, R3/R4. All other adjacencies (R1/R2, R2/R3, R3/R1) are in different areas. So if, say, R2 starts to compute inter-area paths, it will process only LSA-3 received from R4 to construct the routing table. Other LSA-3 received from R1 and R3 will be stored in R2's LSDB but they will be skipped during the SPF run.
Now a host from R4 in area 0 LAN is trying to ping a host in R2 in area 1 LAN.
Will the packets pass through R4-R3-R2 path??
In this case, no, the packets will pass directly through the diagonal from R4 to R2. This is because R4 is directly connected to area 1 and will therefore know the R2 LAN from area 1 as an intra-area route. Intra-area routes are always preferred to inter-area routes.
if you can explain me this process alone. i.e data traffic choosing paths from backbone area in cases where, choosing a route from backbone is less efficient than choosing a route from other area.
You always have to keep the following rules in mind:
- If a router is connected to the area that physically contains the destination network, this network will be known as an intra-area (internal) route to this router. Intra-area routes are always preferred to inter-area routes in OSPF. As a result, even if this router is an ABR and is connected to the backbone area, it will not use the path through the backbone (or any other intermediary area) because that path would be an inter-area route. Instead, this router will immediately forward the traffic into the destination area.
- If a router is an ABR but is not connected to the area that contains the destination network, it will know the network as an inter-area network. This information will be delivered to this router by means of LSA-3 that may, depending on the actual network topology, arrive from any area this ABR is connected to. However, the design of OSPF mandates that only LSA-3 received via backbone adjacencies will be processed during the SPF run. Even if LSA-3 received from non-backbone areas were describing shorter paths than LSA-3 received from backbone, this ABR is not allowed to process them. Hence, this router will always forward the traffic through the backbone.
- If a router is an internal router (i.e. not an ABR) to an area that does not contain the destination network, it will know the network as an inter-area network. In this case, it can receive the LSA-3 describing this network only within and via its own area. Hence, in this case, this router will process any LSA-3 it has in its own database. Such a router will forward the traffic through its own area towards the ABR that provides the shortest total path to the destination.
- In any case, the total metric of an inter-area route is given as a sum of the metric indicated in the LSA-3 plus the distance between your router and the ABR in your area that originated this LSA-3. If there are multiple LSA-3 describing the same network (most probably originated by multiple ABRs), you will choose the path with the lowest total metric.
Say the route 1.1.1.0/24 connected to R1 has gone down. Say that update reaches R2 from R3 a little later because some for some reasons the updated LSA about the removal of 1.1.1.0/24 has not reached R3 from R1. And assume there is a link flap between R2 and R3. Since R3 considers this route to be up, it will advertise 1.1.1.0/24 to R2 which previously had removed 1.1.1.0/24 due to an update from R4. So this network, 1.1.1.0/24 will flood again through the entire network.
Here, remember that R1, R2 and R3 may advertise LSA-3 to each other but because R1/R2, R2/R3, R1/R3 adjacencies are not put into area 0 while all these three routers are ABRs, these LSA-3 will be simply ignored during the SPF run. On these routers, only LSA-3 received via backbone adjacencies (in this case, from R4) will be processed.
In addition, RFC 2328 Section 12.4.3 mandates:
Note that only intra-area routes are advertised into
the backbone, while both intra-area and inter-area routes
are advertised into the other areas.
In other words, even if 1.1.1.0/24 is still known as an inter-area network on R2 or R3, they will never advertise this network back to R4. And even if R2 or R3 advertised the 1.1.1.0/24 via non-backbone areas to each other, the SPF would ignore these LSA-3 because they are received via non-backbone adjacencies.
Please feel welcome to ask further!
Best regards,
Peter
