OSPF Concepts and Hello Packets

• An OSPF area's exact topology remains hidden from outside, while maintaining external router connectivity.
• Routers external to an area lack a complete topological map, which decreases OSPF traffic within that area.
• Segmenting an OSPF routing domain results in non-identical LSDBs across all OSPF routers.
• All routers within the same area will have identical area LSDBs.
• Reduced routing traffic due to segmentation leads to lower router memory and resource usage, enhancing scalability.
• A router can operate multiple OSPF processes, each having its own database.
• Routes from one OSPF process cannot be shared with another without redistribution.
• OSPF process numbers are locally significant and don't need to match between routers for them to become neighbors.

OSPF Hello Packets

• OSPF hello packets discover and maintain neighbors, typically sent to the AllSPFRouters address (224.0.0.5).
• Hello packets contain data such as Router ID (RID), authentication options, Area ID, interface address mask, interface priority, hello interval, and dead interval.
• Router ID (RID): A unique 32-bit ID within an OSPF domain for building the topology.
• Authentication options: Ensures secure communication; options include none, clear text, or Message Digest 5 (MD5).
• Area ID: The OSPF area an interface belongs to, represented as a 32-bit number in dotted-decimal (e.g., 0.0.1.0) or decimal (e.g., 256).
• Interface address mask: The network mask for the primary IP address of the interface sending the hello packet.
• Interface priority: Router interface priority for DR elections.
• Hello interval: Time, in seconds, that a router sends hello packets on an interface.
• Dead interval: Time, in seconds, a router waits for a hello from a neighbor before declaring it down.

OSPF Adjacency States

• Down: No information has been received from the neighbor.
• Attempt: Applicable to Non-Broadcast Multi-Access (NBMA) networks; manually configured neighbors are contacted.
• Init: Hello packets have been received, but bi-directional communication hasn't been established.
• 2-Way: Bi-directional communication is established; a router has seen itself in the neighbor's hello packet.
• ExStart: The first state, where routers decide who will be the primary or secondary for LSDB synchronization.
• Exchange: Routers exchange link states using DBD packets.
• Loading: LSR packets are sent to request more recent LSAs discovered (but not received) in the Exchange state.
• Full: Neighboring routers are fully adjacent.

Designated Router and Backup Designated Router

• Multi-access networks (Ethernet, Frame Relay) can cause scalability issues as the number of routers increases.
• More routers lead to additional LSA flooding and increased OSPF neighbor adjacencies.
• Using a Designated Router (DR) simplifies the network topology and reduces the number of adjacencies.
• If a DR fails, a Backup Designated Router (BDR) becomes the new DR; an election then occurs to replace the BDR.
• The BDR also forms full OSPF adjacencies with all OSPF routers on the segment to minimize transition time.