This is part 2 of 2. My shortish summarized version of the OSPF Link State Advertisements, Area Types, Network Types and Timers.

OSPF Link State Advertisements (LSA’s):

LSA	Name	Description	Area
1	Router LSA	Generated by Any router for any/all links	Intra-area
2	Network LSA	Generated by a DR only for DR-used-links	Intra-area
3	Summary LSA	Generated by ABRs to summarize 1 & 2’s	Inter-area
4	ASBR Summary	Generated by ABRs to summarize NH of Type 5	Inter-area
5	External LSA	Generated by ASBR	Domain-wide (other than stub areas)
7	NSSA External	Generated by ASBR specific to NSSA area	NSSA only – Translated to Type 5 for other areas

Type1 – Originated by all routers to tell the other routers in the area about their connected links.

Type2 – Originated by the DR in the area. Simplifies the graph of the area when there are multiple routers on the same segment.

Type3 – Originated by the ABR. Describes routes in other areas.

Type4 – Originated by the ABR. Describes inter-area reachability to a router that performed redistribution.

Type5 – Originated by the ASBR that performs the redistribution. Describes redistributed routes from another protocol.

Type7 – Originated by the ASBR, but ONLY applies to NSSA external areas. (Converted to type 5 LSA when the ABR of the NSSA originates them into area 0)

LSA	Area	Indentifier
1	Intra-area	O
2	Intra-area	O
3	Inter-area	O IA
4	Inter-area	O IA
5	External	E1 or E2
7	External	N1 or N2

OSPF Area Types:

All devices in an area MUST agree on the stub or nssa parameter for adjacencies to establish!!

• stub = 1, 2, 3 + 0.0.0.0 default route (area <X> stub)
• total stub = 1, 2 + 0.0.0.0 default route (area <X> stub no-summary)
• NSSA = 1, 2, 3, 7 (0.0.0.0 default route is not automatically created although you can add it) (area <X> nssa)
• total NSSA = 1, 2, 7 + 0.0.0.0 default route (area <X> nssa no-summary)

Anything that says “total” must get the no-summary on the CLI.

NOTES:

1. If the NSSA ASBR is also an ABR, Type7 LSA’s will be originated into the NSSA and Type5 LSA’s will be originated into Area 0. Type7 LSA origination into the NSSA, in this case, can be suppressed with area <AREA_NUM> nssa no-redistribution.

2. If NSSA has multiple ABR’s only one will perform Type7 to Type5 translation, based on ABR with highest Router-ID.

3. If the Forwarding Address (FA) = 0.0.0.0, it means traffic MUST be sent towards Advertising Router address to reach destination.

4. If the Forwarding Address (FA) for an external prefix is NOT reachable/filtered, the router will NOT install it into the route table nor will it translate Type7 to Type5 LSA’s. This can be “fixed” by area <AREA_NUM> nssa suppress-fa on the ABR, thus setting the FA to itself (0.0.0.0).

OSPF Network Types:

• Broadcast – Multicast Hellos
• Non-broadcast – Unicast Hellos – neighbor statement
• Point-to-Point – Multicast Hellos
• Point-to-Multipoint – Multicast Hellos
• Point-to-Multipoint (Non-broadcast) – Unicast Hellos – neighbor statement

Mixing network types, combinations that will work:

Reason why some network types are not compatible with other is Network Types that use DR election will only be compatible with other types that also elects/requires DR election.

• Broadcast to Broadcast
• Non-Broadcast to Non-Broadcast
• Point-to-Point to Point-to-Point
• Point-to-Multipoint to Point-to-Multipoint
• Broadcast to Non-Broadcast (MUST adjust hello/dead timers to form adjacency)
• Point-to-Point to Point-to-Multipoint (MUST adjust hello/dead timers to form adjacency)

Point-to-(anything) does not use a DR.

(anything)-to-Multipoint adds a /32 route advertisement. The DR and BDR is the only routers that is REQUIRED to peer with all other routers when “Multipoint” networks is used.

Loopbacks are considered host routes – default advertised as /32 (Actual configured subnet can be advertised by: ip ospf network point-to-point)

OSPF Network Type Timers (Hello/Dead):

Anything with “multi” or “non” uses slow timers (30/120), others use fast timers (10/40).