OSPF

When to prefer Open Shortest Path First (OSPF) route over an EIGRP route

The shortest path between two points is always a straight line or in this case it’s the administrative distance. OSPF has an administrative distance of 110 while EIGRP has only 90. Therefore, EIGRP would be picked because of its lower administrative cost.

If you want to use OSPF over EIGRP, then change the administrative distance in router configuration mode.  Whether you want to change the distance for all routes in the area (intra-area), the distance from one area to another (inter-area), or the distance from other routing domains (external), you can aim closer to the distance using this command. Currently they all have a default distance of 110 but you can change it with one easy command in router configuration mode. Here is the full command:

distance ospf {[intra-area dist1] [inter-area dist2] [external dist3]}

Also a really good incentive to use the distance ospf command is when you have multiple OSPF processes, and you might want to prefer internal routes from other external routes.  For more information on OSPF, see the article, OSPF Configuration Management with SNMP documentation.

Four OSPF Router designations and their responsibilities

OSPF Routers have different responsibilities depending on their position and functionality.  Let’s look at them-

The Internal Router – This router exists within an area. It’s responsible for maintaining a current and accurate database of every subnet within an area . It’s also responsible for forwarding data to other networks by the shortest path.

The Backbone Router – This router exists within the backbone area, also called Area 0.  On some routers, it’s also known as Area 0.0.0.0.

The Area Border Router – Also known as ABR, it’s responsible for connecting two or more areas. It holds a full topological database for each area to which its connected and sends LSA updates between the areas.

The Autonomous System Boundary Router – Also known as ABSR, is used to connect to a network or routing protocol outside the OSPF domain. This router should reside in the backbone area.

Four show commands to Verify the OSPF configuration

show ip ospf – provides information about the OSPF process and its details.
show ip ospf database – provides information about the contents of the topological database.
show ip ospf interface – provides information on how OSPF has been configured on each interface.
show ip ospf neighbor – displays all the information  about the relationship that the router has with its neighbors.