The figure above shows a sample network setup with Area 1 separated by the Backbone Area 0. It is legal to have multiple discontiguous non-zero areas with the same Area ID, and it is not necessary to implement a virtual link or GRE tunnel to setup them in a contiguous manner.
Below shows the routing table and OSPF LSDB on RT2:
RT2#sh ip route Gateway of last resort is not set 20.0.0.0/24 is subnetted, 1 subnets C 20.20.20.0 is directly connected, Serial0/1 40.0.0.0/24 is subnetted, 1 subnets O IA 40.40.40.0 [110/128] via 20.20.20.2, 00:00:36, Serial0/1 10.0.0.0/24 is subnetted, 1 subnets C 10.10.10.0 is directly connected, Serial0/0 30.0.0.0/24 is subnetted, 1 subnets O IA 30.30.30.0 [110/128] via 10.10.10.1, 00:00:36, Serial0/0 RT2# RT2#sh ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 3.3.3.3 0 FULL/ - 00:00:37 20.20.20.2 Serial0/1 1.1.1.1 0 FULL/ - 00:00:37 10.10.10.1 Serial0/0 RT2# RT2#sh ip ospf database OSPF Router with ID (2.2.2.2) (Process ID 100) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 47 0x80000002 0x001144 2 2.2.2.2 2.2.2.2 50 0x80000001 0x00F637 4 3.3.3.3 3.3.3.3 46 0x80000002 0x004ABD 2 Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 30.30.30.0 1.1.1.1 48 0x80000001 0x00B1EC 40.40.40.0 3.3.3.3 48 0x80000001 0x000C6C RT2#
Below shows the routing table and OSPF LSDB on RT1:
RT1#sh ip route Gateway of last resort is not set 20.0.0.0/24 is subnetted, 1 subnets O 20.20.20.0 [110/128] via 10.10.10.2, 00:00:35, Serial0/1 40.0.0.0/24 is subnetted, 1 subnets O IA 40.40.40.0 [110/192] via 10.10.10.2, 00:00:35, Serial0/1 10.0.0.0/24 is subnetted, 1 subnets C 10.10.10.0 is directly connected, Serial0/1 30.0.0.0/24 is subnetted, 1 subnets C 30.30.30.0 is directly connected, Serial0/0 RT1# RT1#sh ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 0 FULL/ - 00:00:30 10.10.10.2 Serial0/1 4.4.4.4 0 FULL/ - 00:00:38 30.30.30.2 Serial0/0 RT1# RT1#sh ip ospf database OSPF Router with ID (1.1.1.1) (Process ID 100) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 44 0x80000002 0x001144 2 2.2.2.2 2.2.2.2 49 0x80000001 0x00F637 4 3.3.3.3 3.3.3.3 45 0x80000002 0x004ABD 2 Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 30.30.30.0 1.1.1.1 45 0x80000001 0x00B1EC 40.40.40.0 3.3.3.3 47 0x80000001 0x000C6C Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 49 0x80000001 0x00DEF6 2 4.4.4.4 4.4.4.4 46 0x80000002 0x009C2C 2 Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum 10.10.10.0 1.1.1.1 45 0x80000001 0x008456 20.20.20.0 1.1.1.1 35 0x80000001 0x009DDE 40.40.40.0 1.1.1.1 35 0x80000001 0x004DB2 RT1#
When an ABR builds LSAs for area 0, it takes the routing information from each of its other areas and includes them into Type-3 Summary LSAs, which do not contain any area information. Therefore, other backbone routers simply don’t know what areas these destinations are belong to. The routers only know that in order to reach these destinations, the next hop is a given ABR.
Below shows the routing table and OSPF LSDB on RT3:
RT3#sh ip route Gateway of last resort is not set 20.0.0.0/24 is subnetted, 1 subnets C 20.20.20.0 is directly connected, Serial0/0 40.0.0.0/24 is subnetted, 1 subnets C 40.40.40.0 is directly connected, Serial0/1 10.0.0.0/24 is subnetted, 1 subnets O 10.10.10.0 [110/128] via 20.20.20.1, 00:00:38, Serial0/0 30.0.0.0/24 is subnetted, 1 subnets O IA 30.30.30.0 [110/192] via 20.20.20.1, 00:00:38, Serial0/0 RT3# RT3#sh ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 0 FULL/ - 00:00:36 20.20.20.1 Serial0/0 5.5.5.5 0 FULL/ - 00:00:33 40.40.40.2 Serial0/1 RT3# RT3#sh ip ospf database OSPF Router with ID (3.3.3.3) (Process ID 100) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 49 0x80000002 0x001144 2 2.2.2.2 2.2.2.2 53 0x80000001 0x00F637 4 3.3.3.3 3.3.3.3 47 0x80000002 0x004ABD 2 Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 30.30.30.0 1.1.1.1 51 0x80000001 0x00B1EC 40.40.40.0 3.3.3.3 48 0x80000001 0x000C6C Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count 3.3.3.3 3.3.3.3 53 0x80000001 0x002C59 2 5.5.5.5 5.5.5.5 51 0x80000002 0x005824 2 Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum 10.10.10.0 3.3.3.3 39 0x80000001 0x00CAC7 20.20.20.0 3.3.3.3 49 0x80000001 0x00DED5 30.30.30.0 3.3.3.3 39 0x80000001 0x007A9B RT3#
Below shows the routing table and OSPF LSDB on RT4 and RT5:
RT4#sh ip route Gateway of last resort is not set 20.0.0.0/24 is subnetted, 1 subnets O IA 20.20.20.0 [110/192] via 30.30.30.1, 00:00:40, Serial0/0 40.0.0.0/24 is subnetted, 1 subnets O IA 40.40.40.0 [110/256] via 30.30.30.1, 00:00:40, Serial0/0 10.0.0.0/24 is subnetted, 1 subnets O IA 10.10.10.0 [110/128] via 30.30.30.1, 00:00:42, Serial0/0 30.0.0.0/24 is subnetted, 1 subnets C 30.30.30.0 is directly connected, Serial0/0 RT4# RT4#sh ip ospf database OSPF Router with ID (4.4.4.4) (Process ID 100) Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 56 0x80000001 0x00DEF6 2 4.4.4.4 4.4.4.4 51 0x80000002 0x009C2C 2 Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum 10.10.10.0 1.1.1.1 52 0x80000001 0x008456 20.20.20.0 1.1.1.1 41 0x80000001 0x009DDE 40.40.40.0 1.1.1.1 41 0x80000001 0x004DB2 RT4# ====================================================================== RT5#sh ip route Gateway of last resort is not set 20.0.0.0/24 is subnetted, 1 subnets O IA 20.20.20.0 [110/128] via 40.40.40.1, 00:00:44, Serial0/0 40.0.0.0/24 is subnetted, 1 subnets C 40.40.40.0 is directly connected, Serial0/0 10.0.0.0/24 is subnetted, 1 subnets O IA 10.10.10.0 [110/192] via 40.40.40.1, 00:00:42, Serial0/0 30.0.0.0/24 is subnetted, 1 subnets O IA 30.30.30.0 [110/256] via 40.40.40.1, 00:00:42, Serial0/0 RT5# RT5#sh ip ospf database OSPF Router with ID (5.5.5.5) (Process ID 100) Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count 3.3.3.3 3.3.3.3 58 0x80000001 0x002C59 2 5.5.5.5 5.5.5.5 54 0x80000002 0x005824 2 Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum 10.10.10.0 3.3.3.3 43 0x80000001 0x00CAC7 20.20.20.0 3.3.3.3 53 0x80000001 0x00DED5 30.30.30.0 3.3.3.3 43 0x80000001 0x007A9B RT5#
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