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"Example of a split OSPF backbone"

Author: David Bombal
CCIE #11023, CCSI, CCDP, CCIP, CCNP, CCSP, CCVP

Overview:

In OSPF, it is a requirement that all areas connect to area 0. It is also a requirement that Area 0 be contiguous in OSPF. These requirements sometimes force the use of OSPF virtual links. A demonstration of this is covered in this document.

Our Diagram:

Notice in the diagram that Area 0 is split by Area 2 and that R4 has another area, Area 40 connected to it. This is going to require that two virtual links are created on R2 and R3 - one pointing to each other and the other to R4

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R2 config:
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router ospf 1
log-adjacency-changes
area 2 virtual-link 4.4.4.4
area 2 virtual-link 3.3.3.3
network 2.2.2.2 0.0.0.0 area 20
network 10.1.1.2 0.0.0.0 area 0
network 10.1.2.1 0.0.0.0 area 2

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R3 config:
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router ospf 1
log-adjacency-changes
area 2 virtual-link 4.4.4.4
area 2 virtual-link 2.2.2.2
network 3.3.3.3 0.0.0.0 area 30
network 10.1.3.2 0.0.0.0 area 2
network 10.1.4.2 0.0.0.0 area 0

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R4 config:
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router ospf 1
log-adjacency-changes
area 2 virtual-link 3.3.3.3
area 2 virtual-link 2.2.2.2
network 4.4.4.4 0.0.0.0 area 40
network 10.1.2.2 0.0.0.0 area 2
network 10.1.3.1 0.0.0.0 area 2

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R1 output:
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r1#traceroute 5.5.5.5

Type escape sequence to abort.
Tracing the route to 5.5.5.5

1 10.1.1.2     16 msec 16 msec 16 msec
2 10.1.2.2     32 msec 32 msec 32 msec
3 10.1.3.2     48 msec 44 msec 44 msec
4 10.1.4.1     60 msec * 56 msec
r1#

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To your success,

David Bombal
Managing Director
Network Experts Limited
david@ConfigureTerminal.com