[j-nsp] Junos OSPF Inter-Area Routes !

[Per Granath]

> Recently I was working on a scenario in OSPF for checking  the use of
> Backbone Area 0 for Inter-Area communication and I was surprised to see
> that 2 Non-Backbone areas were able to exchange inter-area routes without
> any Area 0 configured. Well the same thing did not work in Cisco which was
> the expected behavior per documentations.
> 
> Topology was simple
> 
> R1-Area3-R2-Area2-R1
> 
> Can anyone help to understand the above behavior in Junos.

The Junos ABR will create a type 3 LSA for all connected areas, so you will see the inter-area routes.

Now, if in one area you add another ABR, and that ABR also connect to area 0, there is a loop detection mechanism that will make that ABR only process type 1 and 2 LSAs in the other areas - and use type 3 LSAs only in area 0. All this happens in the SPF algorithm, while the LSA may still be in the database.

(The Juniper course AJSPR has a chapter on it.)