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):
|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)
|5||External||E1 or E2|
|7||External||N1 or N2|
OSPF Area Types:
All devices in an area MUST agree on the
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.
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.
If NSSA has multiple ABR’s only one will perform Type7 to Type5 translation, based on ABR with highest Router-ID.
If the Forwarding Address (FA) = 0.0.0.0, it means traffic MUST be sent towards Advertising Router address to reach destination.
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-faon the ABR, thus setting the FA to itself (0.0.0.0).
OSPF Network Types:
- Broadcast – Multicast Hellos
- Non-broadcast – Unicast Hellos –
- Point-to-Point – Multicast Hellos
- Point-to-Multipoint – Multicast Hellos
- Point-to-Multipoint (Non-broadcast) – Unicast Hellos –
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