IP protocol 89
Flood LSA’s to all OSPF routers using multicast address 224.0.0.5
Flood LSA’s to all OSPF DR / BDR routers using address 224.0.0.6
Designated Router Election – Highest configured priority (128 default), second is using router ID
Router ID – If the RID is not manually set, the address of the primary interface of the router is chosen, usually the loopback interface.
LSA Types
| Type 1 – Router Tells the other routers about directly connected links |
| Type 2 – Network When routers are in a multiaccess network (think ethernet connection with DR election), the lead router, the “designated router” or DR tells other routers about the networks |
| Type 3 – Summary (Tricky name not actually summarized routes) When LSA Type 1 & 2 are re-advertised into another area, they are converted to type 3 “summary” routes. These type 3’s would also be re-advertised into any areas that they didn’t source from. |
| Type 4 – ASBR Summary This one is a bit confusing, it advertises the route to an ASBR router, it is only needed when the ASBR isn’t directly connected to a specific area. |
| Type 5 – Autonomous system external An ASBR tells other routers about a route redistributed from outside OSPF, gets re-advertised to all (normal) areas. |
| Type 6 – Multicast |
| Type 7 – NSSA In an NSSA, this type of LSA gets generated instead of a type 5 route, then gets converted to a type 5 LSA when going into another area. |
| Type 8 – External attribute for BGP |
Area Types
Backbone area (area 0)
Standard area
Stub area – No type 4 or 5 LSA’s (no ext routes from other areas), only type 1,2,3 LSA’s allowed, doesn’t flood external routes to other areas, if manually configured default route can be injected into it by ABR.
Totally stubby area – Same as stub area, but doesn’t allow type 3 LSA (summary routes from other areas), still allows the use of a manually configured default route that can be injected by the ABR.
Not-so-stubby area (NSSA) – Same as stub area, but allows external routes (as type 7 LSA instead of 5), the ASBR then converts these type 7 LSA’s to type 5 LSA’s to advertise the external route into other areas.
Packet Types
| Type 1 – Hello | Sent by each router to form and maintain adjacencies with its neighbors |
| Type 2 – Database Description | Used by the router during the adjacency formation process. It contains the header information for the contents of the LSDB on the router. |
| Type 3 – Link-State Request | Used by the router to request an updated copy of a neighbor’s LSA. |
| Type 4 – Link-State Update | Used by the router to advertise LSA’s into the network. |
| Type 5 – Link-State Acknowledgment | Used by the router to ensure the reliable flooding of the LSA’s throughout the network. |
Adjacency States
Requirements for Adjacency
- Interface types must match (Multi Access / P2P)
- Network mask (On Multi Access only)
- Hello interval
- Dead interval
- Area types
- Area Numbers
- Authentication
- Router ID’s CAN’T match