OSPF – Open Shortest Path First

Open Shortest Path First is a routing protocol for Internet Protocols networks by Interior Gateway Protocol. In mid- 1980s RIP (Routing Information Protocol) was increasingly incapable of serving large, diverse network, for that Internet Engineering Task Force created OSPF based on shortest Path First (SPF) algorithm  

If there are several routers in network, OSPF build a table of the router connection. When data is send from one location to another, SPF or Dijkstra algorithm compares the available options and chooses the most efficient way for the data to be sent. This limits the unnecessary delays in data transmission and prevents infinite loops.

Why OSPF?

Compared to other protocols for routing, there are major reasons to select OSPF. Firstly, there is ease of configuration and availability in a wide range of routers, most routers support it as it is an open standard. As it is link-state route protocol used for allowing routers to learn routes dynamically from other router types and for advertising the route it finds to other router type as well. OSPF is a link-state routing protocol that calls for the sending of link-state advertisement (LSAs) which is summarized route information to all other routers within the same hierarchical area.

OSPF uses “Cost” as the value of metric and uses a Reference Bandwidth of 100 Mbps for cost calculation.

The formula to calculate the cost is Reference Bandwidth divided by Interface Bandwidth. For example, in the case of 10 Mbps Ethernet,

                                OSPF metric cost value is 100 Mbps/ 10 Mbps = 10.

Following table lists OSPF default cost values for some common interface bandwidth,

Bandwidth	OSPF Cost
100 Gbps	1
40 Gbps	1
10 Gbps	1
1 Gbps	1
100 Mbps	1
10 Mbps	10
1.544 Mbps	64
128 Kbps	781

OSPF Basic Operation:

1. OSPF sends Hello Packets out of all OSPF-enabled interfaces. If certain parameters in Hello Packets match between the neighboring routers then they form the relationship called the Adjacency.
2. Then each router exchange special packets called Link State Advertisements contain details such as address/network masks configured on the link, the metric, the state of link (which is its relation to the rest of network), list of neighbors connected.
3. Each router stores the LSAs in its Link State Database (LSDB).
4. Each router runs Dijkstra algorithm to select the best path with loop free using LSDB. The best path stores in routing table.

An OSPF network is divided into areas that are logical groupings of host and networks.

• Backbone Area – The backbone area (known as area 0) forms the core of an OSPF network. All other areas are connected to it and inter-area routing happens via routers connected to the backbone area and to their own associated areas.
• Stub Area - A stub area is an area which does not receive route advertisements external to the AS (Autonomous System-a collection of OSPF areas amongst which OSPF routing information is shared) and routing from within the area is based entirely on a default route of 0.0.0.0.
  
• Not-So-Stubby Areas – These areas are derived from the OSPF stub areas. The motivation behind is to allow OSPF Stub areas to carry external routes (routes learned using other routing protocols like RIP, EIGRP, BGP etc).
• Totally Stubby Area – Totally Stubby Areas protect internal routes by minimizing the routing table and summarizing everything outside the area with a default route. This is a proprietary Cisco solution. It keeps the topological databases and routing table as small as possible.

OSPF router Types:

When an OSPF AS is divided into areas, the routers are classified by one or more of categories defined in table,

Router Type	Description
Internal Router	A router with all interfaces connected to the same area. Internal routers each have a single LSDB.
Area Border Router (ABR)	A router with interfaces connected to different areas. ABRs have multiple LSDBs, one for each attached area.
Backbone Router	A router with an interface on the backbone area. This includes all ABRs and internal routers of the backbone area.
AS Boundary Router(ASBR)	A router that exchanges routes with sources outside of the OSPF AS. ASBRs advertise external routes throughout the OSPF AS.

 Based on the network type, OSPF router can elect one router to be a Designated Router (DR) and one router to be a Backup Designated Router (BDR). For example, on multi-access broadcast networks (Such as LANs) routers defaults to elect a DR and BDR. DR and BDR serve as the central point for exchanging OSPF routing information. Non-BDR router will exchange routing information with the DR and BDR. Instead of exchanging updates with every router on the network segment, DR will distribute topology information to every router. This greatly reduces OSPF traffic.

 
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