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Backbone area and virtual links
Each AS has a backbone area that distributes routing information between non-backbone areas.
Routing information between non-backbone areas must be forwarded by the backbone area. OSPF
has the following requirements:
All non-backbone areas must maintain connectivity to the backbone area.
The backbone area must maintain connectivity within itself.
In practice, these requirements might not be met due to lack of physical links. OSPF virtual links can
solve this issue.
A virtual link is established between two ABRs through a non-backbone area. It must be configured
on both ABRs to take effect. The non-backbone area is called a transit area.
As shown in Figure 18, Area 2 has no direct physical link to the backbone Area 0. You can configure
a virtual link between the two ABRs to connect Area 2 to the backbone area.
Figure 18 Virtual link application 1
Virtual links can also be used as redundant links. If a physical link failure breaks the internal
connectivity of the backbone area, you can configure a virtual link to replace the failed physical link,
as shown in Figure 19.
Figure 19 Virtual link application 2
The virtual link between the two ABRs acts as a point-to-point connection. You can configure
interface parameters, such as hello interval, on the virtual link as they are configured on a physical
interface.
The two ABRs on the virtual link unicast OSPF packets to each other, and the OSPF routers in
between convey these OSPF packets as normal IP packets.
Stub area and totally stub area
A stub area does not distribute Type-5 LSAs to reduce the routing table size and LSAs advertised
within the area. The ABR of the stub area advertises a default route in a Type-3 LSA so that the
routers in the area can reach external networks through the default route.
To further reduce the routing table size and advertised LSAs, you can configure the stub area as a
totally stub area. The ABR of a totally stub area does not advertise inter-area routes or external
Area 0
Area 1
Area 2
ABRABR
Transit area
Virtual link
Area 0
Area
1
Virtual link
R2
R1
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