Role Of The Backbone Area - Juniper J2300 User Manual

J-series™ Services Router User Guide
Figure 64: Multiarea OSPF Topology
Area 1
Area border routers are responsible for sharing topology information between
areas. They summarize the link-state records of each area and advertise destination
address summaries to neighboring areas. The advertisements contain the ID of the
area in which each destination lies, so that packets are routed to the appropriate
area border router. For example, in the OSPF areas shown in Figure 64, packets sent
from router A to router C are automatically routed through area border router B.

Role of the Backbone Area

An OSPF restriction requires all areas to be directly connected to the backbone
area so that packets can be properly routed. All packets are routed first to
the backbone area by default. Packets that are destined for an area other
than the backbone area are then routed to the appropriate area border router
and on to the remote host within the destination area.
In large networks with many areas, in which direct connectivity between
all areas and the backbone area is physically difficult or impossible, you
can configure virtual links to connect noncontiguous areas. For example,
Figure 65 shows a virtual link between a noncontiguous area and the
backbone area through an area connected to both.
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OSPF Overview
C
B
A
Area 0
Area 2