Inter-Area Loop Prevention in OSPF

Question

I found a document on internet that will give some idea on OSPF ABR and Loop Prevention. In that document, they mention This:

1. ABR expects summary LSAs from Area 0 only. This means there should be at least one adjacency in FULL state built over Area 0 interface. In case if ABR has such adjacency, it will ignore summary-LSAs received over non-backbone areas. These LSAs will be installed in the database, but not used for SPF calculations.
2. ABR will accept and use summary-LSAs learned over non-backbone area if it DOES NOT have a FULL adjacency built over an Area 0 interface. It is safe to do so, since the ABR will not be able to flood the summary back into Area 0 creating routing loops.

Can someone give topology that contradicts to point 1? According to my knowledge, in all the cases ABR will establish FULL neighbor relationship to some other "Router" (P2P) or "DR" (Broadcast) in Area0. Why would they specifically mention point 1. Point 2 make sense because of Point 1

Answer 1

Imagine the following network.

There are two ABRs (ABR1 and ABR2). Both ABRs have an interface in area 0 and an interface in area 1. Both adjacencies on both ABRs are in the FULL state. Each ABR will create the following type 3 summary LSAs:

• A. Any network addresses within the area 1 LSDB (type 1 and type 2 LSAs) will be summarised from area 1 into area 0.

• B. Any network addresses within the area 0 LSDB (type 1 and type 2 LSAs) will be summarised from area 0 into area 1.

• C. Any summary LSAs already within area 0 (the result of another ABR generating type 3 LSA into area 0) will be regenerated into area 1. This will involve generating a new type 3 with an updated cost and this LSA will live in the area 1 LSDB.

For A and B, each ABR will now have two sources of information for each network. There will be the original type 1 or type 2 LSA and also a type 3 LSA. The order of precedence is to prefer type 1 and type 2 over type 3, so only the type 1 or type 2 LSA is used in the routing calculation.

For C, the ABR will have the original type 3 (within area 0 LSDB) and it will also have a type 3 received within area 1. In this case, it will ignore the LSA from within area 1 and will only use the LSA from the backbone for its calculations.

Answer 2

A non-backbone area should not generate summary LSAs. What number 1 is saying is that any summary LSA coming from a non-backbone area is ignored. An ABR will have at least one interface in Area 0 (backbone), and at least one interface in a non-backbone area. The ABR only uses summary LSAs received from Area 0.

You could have an area, e.g. Area 1, which has multiple ABRs. Area 1 will get summary LSAs from Area 0 through all the ABRs, and the Area 1 routers will send those summary LSAs to all the routers in Area 1, including all the ABRs. Remember, the ABRs have interfaces in Area 1, too, and are Area 1 routers for those interfaces. The ABRs will not use the summary LSAs coming from the Area 1 interfaces.