- Packages Hello (type = 1): establish and maintain relationships between neighbors.
- Database Description (DBD): (type = 2) describe the contents of the DB.
- Link-State Request (LSR): (type = 3) request packets portions of the DB.
- Link-State Update (LSU) : (type = 4) response packets to portions of the DB.
- Link-State ACK (LSAck) : (type = 5) packages that recognize lsuser.
- Link-State Advertisement (LSA): describe the local state of a router or network. For a router includes the status of the interfaces and adjacencies. Are packaged in DBD, LSU, or LSAck LSR.
Types of LSAs in a multi-area OSPF
- Router LSA: generated by each router in their area to all routers in the area. Network LSA
- : generated by each DR, describes the set of routers connected to the BMA and only ship within the area. Summary LSA
- : generated by ABRs describing routes outside the area (that have come through the backbone) but belonging to AS.
- ASBR summary LSA : generated by the ABR, describes routes to ASBR to let everyone know to go outside.
- AS external LSA: generated by the described ASBR AS external routes.
network in multi-area intra-area routing is common. When the DB are synchronized, the ABR has a topological map of each area and can generate LSAs to other ABR.
The ABR generates a summary LSA for each area network (you can use to optimize summarization). So each ABR router has complete topology of the backbone area and a summarization of each area.
OSPF in different topologies
1) BMA (Broadcast Multi-Access) is the standard case (LANs). The Hello protocol is used for:
- Check that the line with a neighbor is operational and can exchange information.
- elect the DR and BDR.
The hello is sent periodically (every 10 sec) using the multicast address 224.0.0.5.
As mentioned above, each network segment BMA will have a DR and BDR. A router may act as DR on a router segment and act as normal in another, it's actually the interface that acts as a DR or BDR. Keep in mind that it is recommended that a router is DR on several LANs as the CPU consumption is high.
A BDR DR detected a fault for some time because you will not receive LSAs.
During route discovery, it goes through several stages:
- Exstart State: DR and BDR form an adjacency with each of the routers (master-slave). Exchange
- State: exchange DB. Loading
- State: slave compares the information received and ask them to send entries that do not have. Full
- State: establishing a routing table.
Note: multicast address 224.0.0.5 for all OSPF routers. 224.0.0.6 multicast to all DR and BDR routers.
2) Point to point:
two routers in this topology are adyacences by definition and there is no concept of DR and BDR.
3) NBMA (Non-Broadcast Multi-Access): topologies Frame Relay, ATM, etc. In these topologies
no broadcast capabilities there are difficulties in the choice of BR and BDR. There are two methods:
- NMBA: emulates the operation of OSPF on a broadcast network. It is used in fully-meshed networks. For this, the router creates a packet sent by each virtual link listed in the neighbor table. Useful for networks with few neighbors.
- Point-to-Multipoint: each link is treated as if it were a link point to point (no DR and BDR). It is used in partial mesh.
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