How the OSPF works

OSPF FeaturesOSPF (Open Shortest Path First) is a routing protocol that can be used f...

In order to understand how OSPF works, it is first important to have a firm grasp of the overall OSPF process flow

The router ID is a 32-bit number that identifies the OSPF router. Just like IPv4 addresses, it's an 8-bit decimal number separated by ". (dot)" and then write four of them in a row.

In OSPF, each router is identified by its router ID. For this reason, router IDs must not be duplicated. In this section, we will explain what happens if a router ID is duplicated.

There are two types of relationships between OSPF routers: neighbor and adjacency. This section describes the two relationships between OSPF routers.

DR (Designed Router)/BDR (Backup Designated Router) is a router that is elected to efficiently synchronize LSDB on a multi-access network such as Ethernet.

Configure OSPF on the four Cisco routers on the Ethernet and make sure that they are only adjacent with DR.

The OSPF network type is a classification of OSPF enabled interfaces.
This section explains the main network types and what is determined by the network type.

This section describes the LSDB synchronization process between two OSPF routers that are adjacency on Ethernet.

This section describes the issues that arise when OSPF networks become large.

An OSPF area is "a group of OSPF routers with the same LSDB. This section explains the OSPF area in detail.

By dividing the OSPF network into areas, there are different types of OSPF routers. This section describes the different types of OSPF routers.

There are various types of LSAs that are exchanged between OSPF routers.
This section describes the major types of LSAs.

There are several types of OSPF areas, and this section explains the characteristics of the OSPF area types.

There are two basic configuration procedures for OSPF
1. Enable the OSPF routing process.
2. Enable OSPF on the interface

Routing protocols such as OSPF are enabled on the router interface.
This section explains in detail about enabling OSPF on the router interface.

OSPF advertises the route of a loopback interface as a host route.
This section explains in detail about the advertising of loopback interfaces in OSPF.

Describes the commands for configuring and verifying the OSPF Hello/Dead interval on Cisco routers.

The metric for determining the best route in OSPF is the path cost. It is the cumulative cost of an interface on a route.
This section explains the concept of path cost and how to change and check the cost on a Cisco router.

This section describes the commands for configuring and verifying the router priority value to determine DR/BDR on Cisco routers.

OSPF has an authentication feature for neighbors. The authentication feature prevents a router from becoming a neighbor of an unauthorized router.

This section will specifically explain the precautions for neighbor authentication on virtual-link.

This section describes how to configure and verify stub, totaly stub, NSSA, and totaly NSSA on Cisco routers.

Stub area configuration can reduce the number of LSAs advertised to small networks; here is a specific example of stub area configuration on a Cisco router.

You can generate a default route in OSPF and advertise it to other OSPF routers. on Cisco routers, the default-information originate command is mainly used.

By configuring the OSPF area as a stub area, the ABR will automatically generate a default route (0.0.0.0/0).
This section describes a configuration example of default route generation by stub area.

OSPF virtual link is a virtual area 0 point-to-point link. Virtual-link is explained in this section.

This section describes the configuration and verification commands for OSPF virtual-link on Cisco routers.

This is an example of configuring OSPF virtual-link on Cisco routers. The virtual-link provides a virtual connection to the backbone area.

A link failure in the backbone area may result in a discontinuous backbone. In such a case, Virtual-link can be used to configure the backbone area to be continuous.

In OSPF, route summary can be performed with ABR and ASBR.
This section explains the mechanism of route summary with ABR/ASBR in OSPF and the configuration commands on Cisco routers.

This is an example of configuring OSPF inter-area route summary on a Cisco router. Notice the default cost of the default summary route.

When the OSPF route type is different, the priority is determined regardless of the path cost.

This section details how OSPF neighbors can get stuck in the Exstart state.

OSPF defines five types of packets to perform various processes; this section describes the types of OSPF packets and the header formats common to OSPF packets.

OSPF Hello packets are used to establish OSPF neighbors; this section details the format of OSPF Hello packets and the conditions for establishing a neighbor.

OSPF DD packets are packets that provide an overview of LSDB. the format of DD packets is explained.

Request the LSA with OSPF LSR Packet. This section describes the format of LSR packet.

The OSPF LSU packet is used to advertise the LSAs requested in the LSR packet.

OSPF LSAck packet is a packet to confirm the reception of LSAs.

The redistribute maximum-prefix command is used to prevent an unexpectedly large amount of route information from being redistributed to OSPF.
This section describes the redistribute maximum-prefix command.

The route filters in OSPF are LSA type 3 and LSA type 5 filters.
This section provides an overview of LSA type 3/LSA type 5 filters.

This is a configuration example of an OSPF LSA type 3 filter in a simple network diagram. Make sure you have a good grasp of the in and out directions to which the filter will be applied.

This is a configuration example of filtering LSA type 5 in a simple network diagram. We will configure a filter using distribute-list and route-map.

Build an IPv6 routing table by OSPFv3. This is a configuration example for a normal OSPF process that does not support address-family. Also, it takes an area layout that requires virtual links. The basic concept of a virtual link is the same as OSPFv2 for IPv4.

This is a configuration example for summarizing OSPFv3 inter-area routes on Cisco router. The concept of the cost of summarized route is different for OSPFv3 than for OSPFv2 for IPv4.