OSPF Basic

OSPF Operation

• OSPF Router ID
• Designated Router Election
• OSPF Link States
• OSPF Timers
• Neighbor Discovery
• Forming Neighbor Adjacencies
• Link State Advertisements
• Route Summarization
• Shortest Path Algorithm

OSPF Router ID

The OSPF Router ID identifies a specific router in the OSPF topology. The Router ID is either a) the IP address assigned to the loopback interface, or b) the IP address of the interface with the highest IP address number.  Using the loopback interface makes a more stable OSPF environment as the loopback interface is always up, unlike physical interfaces, which can fail. If a physical interface fails, the OSPF router ID may change, triggering router election and link state advertisement flooding.

Designated Router Election

Once the designated router has been chosen, it remains the designated router until it fails.

   I.        Hello interval:  The hello interval is the number of seconds this router waits before sending out the next hello packet.

    II.        Dead interval: The dead-interval is a timer used to time out inactive adjacencies.

Cost: Cost is used by OSPF to set the metric in the router's link state advertisement. The shortest path first (SPF) algorithm uses this value to compute the cost for each route. Routes with lower costs are preferred over routes with higher costs

Neighbor:  Router which are adjacent to each other are said to be neighbors.

Adjacency: Router which are immediate neighbor to each others are said to be adjacent

Hello packet:  It is only through exchange of hello packet two router will become neighbor to each other

Link state advertisement: It contains information regarding the status of the link and the route update