Overview About OSPF | Networking CCNA

OSPF (Open Shortest Path First) is a dynamic routing protocol used in computer networks. The Open Shortest Path First (OSPF) protocol is classified as an Interior Gateway Protocol (IGP). It is utilized to determine the optimal routing path between a source and destination router by implementing the shortest path first (SPF) algorithm. OSPF Protocol is a link-state routing protocol where the routers exchange topology information with their closest neighbors and also use the Dijkstra algorithm to determine the optimal path in the network. It's fast, scalable, and widely used in large networks for its ability to adapt to changes quickly

How OSPF works:

• Neighbor Discovery: Routers running OSPF discover and establish adjacencies with neighboring routers by exchanging Hello packets. This forms neighbor relationships, allowing routers to share routing information.

• Topology Exchange: Once neighbor relationships are established, routers exchange Link State Advertisements (LSAs) containing information about the network topology, including reachable networks and associated costs.

• Building the Topology Database: Each router collects LSAs from neighboring routers and builds a complete map of the network topology, known as the Link State Database (LSDB).

• Shortest Path Calculation: Using the LSDB, each router independently calculates the shortest path to every network in the OSPF domain using the Dijkstra algorithm. This results in a Shortest Path Tree (SPT) rooted at the router.

• Routing Table Generation: Based on the shortest path calculations, routers populate their Routing Tables with entries indicating the best path to each destination network, including the next-hop router and outgoing interface.

• Dynamic Updates and Convergence: OSPF routers continuously monitor the network for changes. Upon detecting a change, such as a link failure or addition, routers quickly update their LSAs and recalculate the shortest paths, ensuring fast convergence and efficient routing in response to network changes.

OSPF makes three type of tables:

1. Neighbor Table: Keeps track of neighboring routers and their states.
2. Topology Table (LSDB): Stores network topology information received from LSAs.
3. Routing Table: Determines the best path to reach destination networks.
   

There are different types of OSPF areas, such as:

1. Backbone Area: The backbone area, also referred to as area0 or area 0.0.0.0, constitutes the fundamental component of an OSPF network, serving as the central point of connection for all other network areas. It distributes routing information among non-backbone area types.
2. Standard area: This is a regular area that can have any area ID other than 0. It can only connect to the backbone area or another standard area through an ABR (Area Border Router).
3. Stub Area: Stud Area relies fully on a default route for its routing needs. This is a special area that does not receive external routes from other AS.
4. Not So Stubby Areas: NSSA can import external AS routes and send them to another area. However, it’s not possible to receive external routes of AS from any other areas.
5. Totally stubby area: This is another variation of the stub area that does not receive any external routes or inter-area routes from other areas. It only has a default route to the backbone area through an ABR.

OSPF Router Types:

1. Internal Routers: All Interfaces are in single area
2. Backbone Routers: At least one interface in Area 0
3. ABR (Area Border Routers): 
• Interfaces are in Area 0 and another Area
• Maintain an LSDB for each Area
• Summarize LSAs between Areas
4. ASBR (Autonomous System Border Routers): Redistributing foreign routes in OSPF

The main states of OSPF are:

• Down: The initial state when a router has no information about a neighbor or a network.
• Init: The state when a router has received a hello packet from a neighbor but has not established bidirectional communication yet.
• Two-way: The state when a router has established bidirectional communication with a neighbor but has yet to decide whether to exchange routing information or not. And the DR and BDR election processed.
• DR and BDR elections – DR and BDR elections are done in broadcast or multi-access networks. Selection criteria are given below:
  • The higher the router priority of a router, the higher priority, DR it will be declared.
  • In router priority, the larger route should be considered if there is a tie. (Then router id or overactive IP address on the router’s interface is considered when no loopback is configured. Otherwise, the highest loopback address is considered before the overactive IP address).
• Exstart: The state when a router has decided to exchange routing information with a neighbor and has negotiated the master-slave relationship and the initial sequence number.
• Exchange: The state when a router exchanges DBD packets with a neighbor to synchronize their LSDBs.
• Loading: The state when a router requests and receives more details about LSAs from a neighbor using LSR, LSU, and LSAck packets.
• Full: The final state when a router has synchronized its LSDB with a neighbor and is ready to forward packets.

There are five types of messages used in OSPF Protocol –

1. Hello: The Hello is used to create neighborhood relationships and analyze the proximity of neighbors. It means that “Hello” is necessary to establish a connection between routers.
2. Database description: After making a connection, when the neighboring router wants to communicate with the system for the first time. It transmits the information to the database for network topology to the system; through this, the system can update or make changes accordingly.
3. Link State Request: The router sends a link-state request to obtain information about the specified route. For example, Router 1 wants information about Router 2, so Router 1 shares a link-state request with Router 2. If Router 2 receives a link-state request, it sends link-state information to Router 1.
4. Link State Update: The router uses link-state updates to announce the status of the link. When a route needs to broadcast the state of its link, it uses link-state updates.
5. Link-State Acknowledgment: With link-state acknowledgment, routing is more reliable by forcing each node to share an acknowledgment on each link-state update. For example, Router A shares a link-state update with Routers B and C. In response, Router B and C share a link-state acknowledgment to Router A, thereby notifying Router A that both routers have received the link-state update.

OSPF has several benefits that make it a popular routing protocol, such as:

1. It can scale to large and complex networks by using hierarchical design, areas, and route summarization.
2. It supports variable-length subnet masking (VLSM) and classless inter-domain routing (CIDR), which allow more efficient use of IP addresses and reduce the size of routing tables.
3. Quicker detection and restoration from a link or node failures are made possible by OSPF’s support for fast reroute (FRR) and bidirectional forwarding detection (BFD).
4. It can support different types of networks, such as broadcast, non-broadcast, point-to-point, point-to-multipoint, and virtual links.
5. It supports different types of traffic, such as unicast, multicast, and anycast.
6. It is an open standard that is widely implemented by different vendors and devices.

Here are the three OSPF intervals with their default values:

1. Hello Interval:

   • Default: 10 seconds on broadcast and point-to-point links.
   • Default: 30 seconds on non-broadcast multi-access (NBMA) networks.

2. Dead Interval:

   • Default: Four times the Hello Interval.
   • Default: 40 seconds on broadcast and point-to-point links.
   • Default: 120 seconds on NBMA networks.

3. Retransmit Interval:

   • Default: 5 seconds.
   • This interval defines how often OSPF routers retransmit LSAs (Link State Advertisements) that have not been acknowledged by neighbors.

These intervals are crucial for neighbor discovery, failure detection, and maintaining the consistency of OSPF routing information.

OSPF (Open Shortest Path First) supports different network types, Each OSPF network type has its own considerations for neighbor relationships, DR/BDR election, and routing updates, ensuring flexibility and scalability in OSPF deployments. Here's the network types:

1. Point-to-Point (P2P): Direct link between two routers, automatic adjacency formation.

1. Broadcast: Ethernet LANs, elects DR/BDR for reduced control traffic.
2. Non-Broadcast Multi-Access (NBMA): Like Frame Relay, manual adjacencies or Next-Hop resolution.
3. Point-to-Multipoint (P2MP): Multipoint network, no DR/BDR, efficient for hub-and-spoke topologies.
4. Point-to-Multipoint Non-Broadcast (P2MP NBMA): Similar to P2MP, operates over NBMA networks like Frame Relay, requires manual adjacencies.

What included in Hello packet?

1. Router ID: Identity of each router. 32 bit in ip address format but not ip address.
2. Hello Interval: Frequency of periodic hello's
3. Dead Interval: Duration to remember neighbor. Typically 4x of hello interval
4. Neighbors: Neighbors router ID on link. Validates two way reachability.
5. Area ID: OSPF area id, Interface belongs to the area.
6. Authentication Data: Password restricted peering.
7. Network Mask: Subnet mask for link.

Additional Items in hello packet:

1. Area Type: Normal, Stub, NSSA
2. DR
3. BDR
4. Priority: 0-255. Default-1

What will happened if no Router ID declared in the router?

In OSPF (Open Shortest Path First), the Router ID (RID) is a crucial identifier for each OSPF router. If no Router ID is explicitly configured, OSPF will automatically select one based on the following criteria:

1. Highest IP Address on a Logical/Loopback Interface:

   • If the router has one or more loopback interfaces configured with IP addresses, OSPF will choose the highest IP address among those loopback interfaces as the Router ID.

2. Highest IP Address on a Physical Interface:

   • If no loopback interfaces are configured, OSPF will choose the highest IP address on any of the router's active physical interfaces.