-ABRs do not forward Type 1 and Type 2 LSAs from one area into another area.
-OSPF advertises interarea routes using the Type 3 summary LSA. ABRs generate a Type 3 LSA for each subnet in one area, and advertises each Type 3 LSA into the other areas.
-Type 3 summary LSAs do not contain all the detailed topology information, so in comparison to Types 1 and 2, these LSAs summarize the information–hence the name summary LSA.
Router R9 is an ABR between areas 0, 2 and 3, so it creates 3 Type 3 LSAs, one per area:
R9#show ip ospf database
OSPF Router with ID (9.9.9.9) (Process ID 1)
...
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.0.1.1 9.9.9.9 1337 0x80000001 0x00E22A
10.0.1.2 9.9.9.9 1337 0x80000001 0x005B70
10.0.1.3 9.9.9.9 1309 0x80000001 0x005179
10.0.10.10 9.9.9.9 1339 0x80000001 0x00B108
10.0.11.11 9.9.9.9 1310 0x80000001 0x009C1B
192.168.3.0 9.9.9.9 1399 0x80000001 0x00F9AB
192.168.33.33 9.9.9.9 1359 0x80000001 0x007FE2
192.168.168.0 2.2.2.2 1400 0x80000002 0x00AC6E
192.168.169.170 2.2.2.2 1351 0x80000001 0x001557
...
Summary Net Link States (Area 2)
Link ID ADV Router Age Seq# Checksum
192.168.0.0 9.9.9.9 1389 0x80000001 0x0085E6
192.168.0.12 9.9.9.9 1389 0x80000001 0x0043E2
192.168.0.16 9.9.9.9 1390 0x80000001 0x001B07
192.168.0.20 9.9.9.9 1400 0x80000001 0x0070ED
192.168.3.0 9.9.9.9 1400 0x80000001 0x00F9AB
192.168.13.13 9.9.9.9 1350 0x80000001 0x00A7A2
192.168.33.33 9.9.9.9 1366 0x80000001 0x007FE2
192.168.100.100 9.9.9.9 1396 0x80000001 0x00198C
192.168.168.0 9.9.9.9 1356 0x80000001 0x00C2F2
192.168.169.170 9.9.9.9 1356 0x80000001 0x0029DC
...
Summary Net Link States (Area 3)
Link ID ADV Router Age Seq# Checksum
0.0.0.0 9.9.9.9 1411 0x80000001 0x002AE7
Note: Link ID 0.0.0.0 refers to the default route of router R12 in area 3:
R12#show ip route
...
O*IA 0.0.0.0/0 [110/11] via 192.168.3.1, 00:28:59, FastEthernet0/0
Routers which are not ABRs will have information about networks outside their area:
R5#show ip ospf database
OSPF Router with ID (5.5.5.5) (Process ID 1)
...
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.0.1.1 9.9.9.9 276 0x80000002 0x00E02B
10.0.1.2 9.9.9.9 276 0x80000002 0x005971
10.0.1.3 9.9.9.9 276 0x80000002 0x004F7A
10.0.10.10 9.9.9.9 279 0x80000002 0x00AF09
10.0.11.11 9.9.9.9 279 0x80000002 0x009A1C
192.168.3.0 9.9.9.9 279 0x80000002 0x00F7AC
192.168.33.33 9.9.9.9 279 0x80000002 0x007DE3
192.168.168.0 2.2.2.2 293 0x80000003 0x00AA6F
192.168.169.170 2.2.2.2 293 0x80000002 0x001358
R5#show ip ospf database summary 10.0.1.1
OSPF Router with ID (5.5.5.5) (Process ID 1)
Summary Net Link States (Area 0)
Routing Bit Set on this LSA
LS age: 573
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 10.0.1.1 (summary Network Number)
Advertising Router: 9.9.9.9
LS Seq Number: 80000002
Checksum: 0xE02B
Length: 28
Network Mask: /32
TOS: 0 Metric: 0
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