OSPFv2 LSA Notes

OSPF-

The router that generated the LSA, floods a new copy with an incremented sequence number and an age of zero.

LSA Aging time - 60 Minutes

LSA Refresh -     30 minutes

LSA Group Pacing (240 seconds) - This solves the problem of a single timer expiring and a massive update is flooded through the network.

Router OSPF 1 (Depends on the IOS in use)
#timers lsa-group-pacing / #timers pacing lsa-group

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LSA TYPES -

Type 1 - Router LSA - Lists all of the routers links, interfaces, the state, the cost of each link and any known OSPF neighbors on the link. These are flooded only within the area in which they were originated. (#show ip ospf database router)

Type 2 - Network LSA - They are produced by the DR and lists all attached routers including the DR itself. These are only flooded with the originating area. Notice there is no metric as the cost to the DR is always (0) (#show ip ospf database network)

Type 3 - Network Summary LSA - These are originated by ABRs. They are sent into a single area to advertise destinations outside that area. In essence, these are the destinations that the ABR can reach. Default routes external to the area but internal to the OSPF process are also advertised by this LSA type. (#show ip ospf data summary) When an ABR originates an Network Summary LSA, it includes the cost from itself to the destination the LSA is advertising. The ABR will only generate (1) LSA for a given network even if it knows of multiple ways to get there. Although OSPF is a link-state routing protocol within an area, it uses distance vector algorythm to find inter-area routes.

Type 4 - ASBR Summary LSA - These are also generated by the ABR. These are identical to Network LSAs except that the destination that they advertise is an ASBP. (#show ip ospf database asbr-summary) The destination advertised will always be a host route as its a route to a router.

Type 5 - AS External LSA - These are originated by ASBRs. They advertise either a destination external to the OSPF AS or a default-route external to the OSPF AS. AS external are flooded throughout the entire AS. (#show ip ospf database external)

Type 7 - NSSA External LSA - These are originated by ASBRs within a NSSA. These are almost identical to AS external but these are not flooded throughout the entire AS, only the NSSA. (#show ip ospf database nssa-external)

"Routing Bit Set on this LSA" - This is not part of the LSA but an internal maintenance bit indicating that the destination is valid / Is in the routing table.

STUB Areas - A stub area is an area in which no AS External LSAs are flooded. Type (4 - 5) LSAa are not neccasary. ABRs at the edge of a stub area, use Network LSAs or Type - 3 to advertise a single default route into the area.

There are (4) restrictions on "STUB" areas.

1. All stub routers will set a flag (E - Bit) in there hello packets to zero (0) . They will not accept any hellos from a router with the (E - Bit) set to one (1).

2. VLs cant be configured within or transit through a stub area.

3. No router within a stub area can be a ASBR. ASBRs produce type - 5 LSAs and those are not allowed in a stub area.

4. A stub area may have more than one ABR.

Totally Stubby Area - 

These use a default-route to reach destinations external to the AS but also external to the area. Only (1) type 3 LSA is allowed into the area and thats a default route.

Not-So-Stubby Area -

Allows external routes to be advertised into the AS but still retain the characteristics of the stub area. This ASBR will generate Type - 7 LSAs and are flooded throughout the NSSA and blocked by the ABR.

The NSSA External LSA has a flag set known as the (P - Bit). If the NSSAs ABR recieves a type - 7 LSA with the P - Bit set to one (1), it will translate the Type 7 to a Type 5 and flood it throughout the AS. If the (P - Bit) is set to zero (0), no translation will take place and the destinations will not be advertised outside of the NSSA.