Technical Tip: OSPF multiple areas with no backbone

subramanis · ‎10-25-2021

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 <-----