First lets make sure we're comfortable with the LSA types, because you will not understand Stubs before you understand LSAs 100%
*LSA - Link State Advertisement, Each LSA has a LSID (Link State ID, like Router-ID for the LSAs)
LSA 1 - Router LSA, One per Router (Generated by Each Router)
LSA 2 - Network LSA, One per Network (Generated by DR)
LSA 3 - Summary LSA, One per Area (generated by ABR when LSAs 1 and 2 are injected into another Area). LSA3 = Subnet + Mask + Cost to reach the Network
LSA 4 - Summary External LSA, One per Autonomous System (Generated by ASBR)
LSA 5 - External LSA, Injected into OSPF from another routing process (non-ospf)
LSA 6 - Grout Membership LSA, Not supported by Cisco
LSA 7 - NSSA External, Generated by ASBR inside the NSSA instead of LSA 5 (details explained below, NSSA Section)
LSA 8-11 - Not implemented by Cisco
Check the LSA Statistics using the command:
Cisqueros_R3(config-router)#do show ip ospf stat
OSPF Router with ID (3.3.3.3) (Process ID 1)
Area 0: SPF algorithm executed 4 times
Summary OSPF SPF statistic
SPF calculation time
Delta T Intra D-Intra Summ D-Summ Ext D-Ext Total Reason
00:22:26 0 0 0 0 0 0 0 R
00:22:16 0 0 0 0 0 0 0 R
00:21:47 0 0 0 0 0 0 0 R, N, SN
00:20:01 0 0 0 0 0 0 0 R, SN
Check the OSPF DATABASE and all the LSAs currently in it:
Cisqueros_R3#show ip ospf database
OSPF Router with ID (3.3.3.3) (Process ID 1)
Router Link States (Area 0) <--- LSA1
Link ID ADV Router Age Seq# Checksum Link count
2.2.2.2 2.2.2.2 79 0x80000003 0x000E94 2
3.3.3.3 3.3.3.3 78 0x80000007 0x006F2C 4
4.4.4.4 4.4.4.4 52 0x80000004 0x007781 3
Net Link States (Area 0) <--- LSA2
Link ID ADV Router Age Seq# Checksum
10.1.23.3 3.3.3.3 78 0x80000001 0x00658F
Summary Net Link States (Area 0) <--- LSA3
Link ID ADV Router Age Seq# Checksum
1.1.1.0 2.2.2.2 124 0x80000002 0x00B33C
2.2.2.0 2.2.2.2 124 0x80000002 0x000D20
10.1.12.0 2.2.2.2 124 0x80000002 0x00BA22
10.1.45.0 4.4.4.4 43 0x80000001 0x00F5F4
44.4.4.0 4.4.4.4 43 0x80000001 0x008077
Router Link States (Area 1) <--- LSA1, Area 1
Link ID ADV Router Age Seq# Checksum Link count
3.3.3.3 3.3.3.3 89 0x80000007 0x00AC78 0
Router Link States (Area 2) <--- LSA2, Area 1
Link ID ADV Router Age Seq# Checksum Link count
3.3.3.3 3.3.3.3 90 0x80000006 0x00AE77 0
STUB - No LSA 5 (E1 and E2) advertised on ABRs. You cannot use a Virtual Link here, but GRE Tunnel is an option.
Totally Stubby Area - Stub without the Summary Routes (LSA3), so no Intra-Area (IA O). Only Route and Network LSAs (LSA1 and LSA2) within the Area
The "no-summary" attribute is ONLY necessary on ABR, because the ABR is the only router that actually originates the LSA 3.
* STUB and Totally Stubby Area: DO NOT FORGET to configure the "area X stub" on all the routers in the Area X. The routers that ONLY belong to the Area X will BY DEFAULT get the Default Route injected with the ABR as a Default Gateway.
NSSA - LSA3 and LSA7 copied into LSA5 (External, E1 and E2)
* ASBR Generates LSA 7 instead of LSA 5 because the LSA 5 is not supported by NSSA. Then the ABR transforms it into LSA 5 on the ingress from NSSA to the regular OSPF Area.
Totally Stubby NSSA - No Summary (LSA3), but External Routes Included (LSA5)
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