| Internal routers, ABRs, ASBRs, and
backbone routers each play a role in communicating OSPF routing
information in a multiarea network. This section summarizes how the
different types of OSPF routers flood information and how they build
their routing tables when operating within a multiarea environment.
In Chapter 4, you saw that a packet
destined for a network within an area is merely forwarded from one
internal router to another until it reaches the destination network.
However, what if a packet must traverse
multiple areas as shown in the figure?
In this case, the packet must exit
Area 1 via ABR1. ABR1 then sends the packet through the
backbone area to ABR2. Finally, ABR2 can forward the packet to an
internal router in Area 50. The internal router then delivers the
message to the appropriate host on that network.
For the OSPF routers in this example
to make these routing decisions, they must build sufficient routing
tables by exchanging LSUs. The LSU exchange process within a single
OSPF area relies on just two LSA types-Type 1 and Type 2. To
distribute routing information to multiple areas efficiently, Type 3
and Type 4 LSAs must be used by ABRs. The following sections
describe how LSUs containing the various LSA types are flooded to
multiple areas, and how OSPF routers use this information to update
their routing tables.
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