several ABR routers

[ArsenLiliyev]

Hello everyone. There is a couple questions about OSPF topology. As it known, ABR routers - routers which connect its area to zero area. For example, 0,1,2 areas, and there is only 1 ABR router on each area, then there is only 2 ABR could bi in this topology. But could be another connections between other router from 1 area to router from 0? It might work like just simply route, don't send LSA, working like ABR, but it can work like simple route? Or there is no other connection between 1 and 0 but ABR connection? Second question is same, - could it be connection between 1 and 2 and work as simply route, not like ABR, or it is not?

[MHM Cisco World]

it better to draw the topology 

MHM

[M02@rt37 · Accepted solution]

Hello @ArsenLiliyev 

In OSPF, an ABR is responsible for connecting different OSPF areas. A router can be an ABR if it has interfaces in multiple areas, allowing it to perform route summarization and redistribution between those areas.

To answer your questions, in OSPF, all inter-area routing between non-backbone areas (like Area 1) and the backbone area (Area 0) must pass through an ABR. Direct connections between routers in Area 1 and routers in Area 0 without an ABR are not OSPF-compliant. These connections would not function as simple routes; OSPF requires the use of LSAs and the SPF algorithm to calculate routes.

Connection between Area 1 and Area 2 ? Similar to the above, OSPF requires all inter-area routing between non-backbone areas (like Area 1 and Area 2) to pass through an ABR. Direct connections between routers in different non-backbone areas without an ABR are not supported in OSPF. Again, OSPF routing relies on LSAs and the SPF algorithm for proper functioning.

OSPF is designed to use ABRs to connect non-backbone areas to the backbone area and facilitate proper routing between areas. Direct connections between routers in different areas without an ABR are not part of the OSPF protocol and would not function as expected...

[ArsenLiliyev]

Yes, then ABR is router which strictly connected to the backbone area, and there is no ABR between 1 and 2 areas? Connections between 1 and 2 is impossible in OSPF and could be only via ABR connected to the zero area?

[MHM Cisco World]

 ABR connected to the zero area<<-
how OSPF know if this router is ABR or not ?
there is bit, this bit set only if router connect to Zero Area 

so if router connect to multi area but not connect to zero area this router not work as ABR and not generate LSA3

MHM

[liviu.gheorghe]

Except in cases where you cannot connect area 2 to the backbone area, but you can connect it to a router in area 1. In this cases you can configure a Virtual Link between the area 2 and area 0 that will transit area 1. This is only a way to solve a problem and should not be considered as a design practice.

[ArsenLiliyev]

I got it all, thanks everyone!

[M02@rt37]

You're so welcome @ArsenLiliyev 

[Joseph W. Doherty]

BTW a non ABR router, connected to multiple areas will route between them.

What such a router will not do, is inform any area's routers of another area's routes.