OSPF - Propagation of routes between areas

In OSPF, the hierarchy of the backbone and non-backbone areas is necessary due to the way that routes are propagated between OSPF areas.

1st Principle

Any LSAs propagated from a non-backbone area into a backbone area are propagated into all other non-backbone areas.

Take a look at this diagram: ospf-two-areas-multi-area.png

  • LSAs propagated from Area 1 will enter Area 0
  • Those LSAs will be propagated to Area 2
  • If Area 2 and Area 1 were connected, no LSAs would be exchanged between them.

2nd principle

Any LSAs propagated from a backbone area into a non-backbone area are never propagated to any other areas, even if another one of those areas is a backbone area.

Take a look at the following topology:

ospf-virtual-link-topology.png

  • LSAs about the 1.1.1.0/24 network are propagated from Area 0 to Area 1.
  • LSAs about the 1.1.1.0/24 network are not propagated from Area 1 to Area 2.

Take a look at the following topology:

ospf-virtual-link-lab.png

  • LSAs about the 1.1.1.0/24 network are propagated from Area 0 on the left to Area 1
  • LSAs about the 1.1.1.0/24 network are NOT propagated from Area 1 to Area 0 on the right.

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