Why must every area connect to area 0?
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Between areas, OSPF behaves like distance vector: ABRs advertise Type 3 summaries and do not share full topology. To keep that loop free, OSPF forces a hub-and-spoke shape. Area 0 is the hub, every inter-area path crosses it, and ABRs only use summaries learned from the backbone to compute inter-area routes.
Go deeper
The loop-prevention rules
- An ABR advertises into area 0 the intra-area routes of its non-backbone areas.
- An ABR advertises into non-backbone areas the routes it learned from area 0 and its other areas.
- An ABR with a backbone adjacency ignores Type 3 LSAs received in non-backbone areas when it computes inter-area routes. That rule stops a summary from going area 1, to area 2, and back again.
What breaks without it
An area attached only to another non-backbone area gets no inter-area routes, because the ABR in between will not pass summaries from one non-backbone area to another.
Fixes
- Connect the area to area 0 physically.
- A virtual link through the transit area, as a temporary fix.
- A GRE tunnel placed in area 0.
They will ask next
Can area 0 itself be split in two?
Yes, a partitioned backbone. Each half works internally, but inter-area traffic between the halves fails. A virtual link through a non-backbone area can rejoin the halves.
Traps
- Saying area 0 is required because it has the "fastest routers". It is a loop-prevention rule.
How did you do?Book 4.1 Global Internet