Technical Tip: OSPF multiple areas with no backbone

[subramanis]

Description
This article describes that routes learned from the other OSPF areas will be removed on the ABR router when it has multiple areas and has no backbone connection.
Solution
Diagram.

The network 44.44.44.44/32 installed on FGT2 routing table which is advertised from FGT4 and it will be removed from the FG2 routing table when creating another area 0.0.0.2 which has no backbone connection.

FGT2 # show router ospf 

# config router ospf

    set router-id 2.2.2.2

    config area

        edit 0.0.0.1

        next

    end

    # config ospf-interface

        edit "port3"

            set interface "port3"

        next

    end

    # config network

        edit 1

            set prefix 20.0.0.0 255.255.255.0

            set area 0.0.0.1

        next

    end

FGT2 # get router info ospf  neighbor 

OSPF process 0, VRF 0:

Neighbor ID     Pri   State           Dead Time   Address         Interface

3.3.3.3           1   Full/DR         00:00:33    20.0.0.3        port3

The network 44.44.44.44/32 is installed in the routing table

FGT2 # get router info routing-table ospf 

Routing table for VRF=0

O IA    30.0.0.0/24 [110/2] via 20.0.0.3, port3, 00:01:52

O IA    44.44.44.44/32 [110/102] via 20.0.0.3, port3, 00:01:52

Adding another area 0.0.0.2 on FGT2.

FGT2 # show  router ospf 

# config router ospf

    set router-id 2.2.2.2

    # config area

        edit 0.0.0.1

        next

        edit 0.0.0.2               <-----

        next

    end

    # config ospf-interface

        edit "port3"

            set interface "port3"  <----- Attached to 0.0.0.1

        next

        edit "port2"

            set interface "port2"  <----- Attached to 0.0.0.2

        next

    end

    # config network

        edit 1

            set prefix 20.0.0.0 255.255.255.0

            set area 0.0.0.1

        next

        edit 2

            set prefix 10.0.0.0 255.255.255.0

            set area 0.0.0.2

        next

    end

FGT2 # get router info ospf neighbor

OSPF process 0, VRF 0:

Neighbor ID     Pri   State           Dead Time   Address         Interface

1.1.1.1           1   Full/Backup     00:00:35    10.0.0.1        port2

3.3.3.3           1   Full/DR             00:00:33    20.0.0.3        port3

The OSPF neighbourship is established and the network 44.44.44.44/32 is removed from the routing table. it’s in the database table.

FGT2 # get router info routing-table ospf 

No route available

FGT2 # get router info ospf  database brief

OSPF Router with ID (2.2.2.2) (Process ID 0, VRF 0)

                Router Link States (Area 0.0.0.1)

Link ID         ADV Router      Age  Seq#     CkSum Flag Link count

2.2.2.2         2.2.2.2         87   80000004 d53c  0021 1

3.3.3.3         3.3.3.3         92   80000006 9373  0002 1

                Net Link States (Area 0.0.0.1)

Link ID         ADV Router      Age  Seq#     CkSum Flag

20.0.0.3        3.3.3.3         93   80000001 ce54  0002

                Summary Link States (Area 0.0.0.1)

Link ID         ADV Router      Age  Seq#     CkSum Flag Route

10.0.0.0        2.2.2.2         102  80000001 b693  0021 10.0.0.0/24

30.0.0.0        3.3.3.3         325  80000001 939e  0002 30.0.0.0/24

44.44.44.44     3.3.3.3         300  80000001 1724  0002 44.44.44.44/32  <-----

                Router Link States (Area 0.0.0.2)

Link ID         ADV Router      Age  Seq#     CkSum Flag Link count

1.1.1.1         1.1.1.1         20   80000003 54dd  0002 1

2.2.2.2         2.2.2.2         20   80000004 1710  0021 1

                Net Link States (Area 0.0.0.2)

Link ID         ADV Router      Age  Seq#     CkSum Flag

10.0.0.2        2.2.2.2         20   80000001 2515  0021

                Summary Link States (Area 0.0.0.2)

Link ID         ADV Router      Age  Seq#     CkSum Flag Route

20.0.0.0        2.2.2.2         102  80000001 340c  0021 20.0.0.0/24

Solution.
Selecting the ABR type compatible with the routers on your network can reduce or eliminate the need for configuring and maintaining virtual links. For more information, see RFC 3509.

FGT2 # show router ospf 

#cconfig router ospf

    set abr-type cisco             <-----

    set router-id 2.2.2.2

    config area

        edit 0.0.0.1

        next

        edit 0.0.0.2

        next

    end

FGT2 # get router info routing-table ospf

Routing table for VRF=0

O IA    30.0.0.0/24 [110/2] via 20.0.0.3, port3, 00:00:00

O IA    44.44.44.44/32 [110/102] via 20.0.0.3, port3, 00:00:00    <-----