OSPF (Open Shortest Path First) is an open standard link-state routing protocol that runs the Dijkstra Shortest Path First algorithm. A lot of technical information related to OSPF can be found in RFC 2328. When an OSPF link goes up or down a link-state advertisement (LSA) is generated. LSAs are shared with neighbors and put into the link-state database (LDSB) or topology table. OSPF also uses the neighbor, topology and routing tables just like EIGRP. However, unlike EIGRP every time there is a network change the entire routing table is propagated. An OSPF area is used to logically seperate the routers into more manageable groupings. They are also used to lower the amount of resources on each segment router. The backbone or transit area (Area 0) is always the central area and all other areas must attach to the backbone area. The routers that connect an area and the backbone area is known as an Area Border Router (ABR) and a router that connects to other routing domains, usually the internet, are known as an Autonomous System Boundary Router (ASBR).
The following list is all the possible states of a neighbor relationship:
- Down - No hellos have been received
- Attempt - Only used for manual neighbors in NBMA (Nonbroadcast Multiaccess)
- Init - Hello received but router-id was included
- 2-Way - Bi-directional communication has been established
- Exstart - DR (Designated Router) & BDR (Backup DR) have been elected
- Exchange - Routers exchange DBD
- Loading - Exchange of link-state information
- Full - Routers are fully adjacent
The Designated Router maintains adjacencies with all segment routers and the Backup Designated Router is used for redundancy. OSPF has five packet types:
- Hello - used to establish communication with neighbors
- Database Descriptor (DBD) - used to send a list of router IDs
- Link State Requests (LSR)- Follows DBDs to look for missing LSAs
- Link State Update (LSU)- replies to LSRs
- Link State Acknowledgements (LSAck) - confirms receipt of link-state information
There are four different types of OSPF network types:
- Broadcast Multiaccess - Cisco
- Point-to-Point - Cisco
- Point-to-Multipoint Broadcast/Nonbroadcast - RFC Compliant
- Nonbroadcast Multiaccess (NBMA) - RFC Compliant
OSPF supports four different types of areas:
- Standard - regular area
- Stub - Doesn’t allow external summary routes (5)
- Totally Stubby - Doesn’t accept summary LSAs (3,4,5)
- Not-so-Stubby (NSSA) - stub area that allows an ASBR
The following table describes the different types of LSA packets:
| LSA TYPE |
ROUTING TABLE ENTRY |
DESCRIPTION |
| 1 - Router Link |
O |
lists neighbors and their cost |
| 2 - Network Link |
O |
DR lists adjacent routers |
| 3 - Network Summary |
OIA |
summary between ABRs |
| 4 - Summary Link |
OIA |
ASBR advertises itself |
| 5 - External Link |
OE1 or OE2 |
default route out of the AS |
| 7 - NSSA External |
OE1 or OE2 |
Type 5 LSA in a NSSA |
