OSPF

 OSPF (Open Shortest Path First) is a routing protocol used to distribute routing information within a single autonomous system (AS) in a routing domain. It is a classless routing protocol, meaning that it supports variable-length subnet masks (VLSMs), which allows for more efficient use of IP address space.

Here are some key points to remember for CCNA preparation:

• OSPF uses a hierarchical design, where the entire routing domain is divided into areas. This reduces the size of the routing tables and speeds up convergence.
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• OSPF uses a link state routing (LSR) algorithm, which means that each router maintains a database of the state of all links in the network.
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• OSPF routers use a designated router (DR) and a backup designated router (BDR) to reduce the amount of routing information that needs to be exchanged between routers.
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• OSPF uses a cost as the metric to determine the best path to a destination. The cost is inversely proportional to the bandwidth of the link.
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• OSPF supports authentication to prevent unauthorized routers from participating in OSPF routing.
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• OSPFv3 support IPv6 addressing and is similar to OSPFv2

Examples:

• To enable OSPF on a Cisco router

router ospf <process-id>

• To assign an interface to an OSPF area

interface <interface-name>

ip ospf <area-id> area <area-id>

• To check the OSPF routing table

show ip ospf neighbor

OSPF Neighbor States:

• Down: The OSPF process is not running on the neighboring router or the routers are not configured to form an adjacency.
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• Attempt: The routers have begun the process of forming an adjacency, but have not yet completed it.
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• Init: The routers have exchanged hello packets and have agreed on the OSPF options, but have not yet exchanged their full routing tables.
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• 2-Way: The routers have exchanged their full routing tables and have formed a bidirectional adjacency.
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• Exstart: The routers have begun the process of negotiating the master/slave relationship for the exchange of routing information.
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• Exchange: The routers are exchanging routing information.
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• Loading: The routers have completed the exchange of routing information, but have not yet finished calculating the new routing table.
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• Full: The routers have completed the process of forming an adjacency and are fully functional OSPF neighbors.

Area Types:

• Backbone Area (Area 0): All other OSPF areas must connect to the backbone area. It is the central area in the OSPF network and must be contiguous.
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• Stub Area: A stub area is an area that has a default route injected into it by the ABR (Area Border Router) instead of a full routing table.
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• Totally Stubby Area: A totally stubby area is an area where the ABR injects a default route and suppresses external routes.
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• Not-so-stubby Area (NSSA): A NSSA is similar to a stub area, but it allows for the injection of Type 7 external routes by an ASBR (Autonomous System Border Router) within the area.

Router Types:

• Internal Router: A router that is fully adjacent to other routers in the same area.
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• Area Border Router (ABR): A router that is connected to more than one OSPF area. It injects routes from other areas into the area it is connected to.
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• Autonomous System Border Router (ASBR): A router that is connected to the OSPF AS and redistributes routes from other routing protocols into OSPF.

External Types:

• Type 1: External routes that are redistributed into OSPF with a cost equal to the OSPF internal cost to the ASBR.
• Type 2: External routes that are redistributed into OSPF with a cost equal to the cost advertised by the external routing protocol.

LSA Types

OSPF (Open Shortest Path First) is a popular routing protocol that uses Link State Advertisements (LSAs) to propagate information about network topology to all routers in the same area. There are several different types of OSPF LSAs, each of which serves a different purpose:

1. Router LSA (Type 1) - Generated by each router in the area to advertise its links to other networks and routers in the area.

2. Network LSA (Type 2) - Generated by the designated router (DR) on a multi-access network to advertise the presence of the network and the routers attached to it.

3. Summary LSA (Type 3) - Generated by Area Border Routers (ABRs) to advertise routes learned from other areas into a different area.

4. AS-External LSA (Type 5) - Generated by AS boundary routers to advertise external routes learned from other ASes.

5. NSSA External LSA (Type 7) - Generated by Not-So-Stubby Area (NSSA) ABRs to advertise external routes into an NSSA.

What LSA maps to what Area Type :

1. Normal Area - All five LSA types are generated and flooded throughout the area.

2. Stub Area - Only Router LSAs, Summary LSAs and Type 3 Summary LSAs with a metric of "infinity" are generated and flooded.

3. Totally Stubby Area - Only Summary LSAs and Type 3 Summary LSAs with a metric of "infinity" are generated and flooded.

4. Not-So-Stubby Area (NSSA) - All five LSA types are generated and flooded, with the exception of Type 5 External LSAs, which are converted to Type 7 NSSAs and flooded only within the area.