Routing Technologies-OSPF overview

OSPF (Open Shortest Path First) is a connection state routing protocol. Since it is an open standard, it is executed by an assortment of system merchants. OSPF will keep running on most routers that don’t really need to be Cisco routers (not at all like EIGRP which can be run just on Cisco routers).

Here are the most imperative highlights of OSPF:

  • a classless routing protocol
  • underpins VLSM, CIDR, manual route synopsis, level with cost stack adjusting
  • incremental updates are upheld
  • utilizes just a single parameter as the metric – the interface cost.
  • the administrative distance of OSPF courses is, as a matter of course, 110.
  • utilizes multicast addresses and for routing updates.

Routers running OSPF need to build up neighbor connections before trading routes. Since OSPF is a connection state routing protocol, neighbors don’t trade steering tables. Rather, they trade data about network topology. Each OSFP router at that point runs SFP calculation to ascertain the best routes and adds those to the routing table. Since every switch knows the whole topology of a router, the shot for a routing circle to happen is negligible.

Each OSPF router stores routing and topology data in three tables:

Neighbor table – stores data about OSPF neighbors
Topology table – stores the topology structure of a network
Routing table – stores the best routes

OSPF neighbors

OSPF routers need to set up a neighbor relationship before trading routing refreshes. OSPF neighbors are progressively found by sending Hello bundles out each OSPF-empowered interface on a router. Hi, parcels are sent to the multicast IP address of

The procedure is clarified in the accompanying figure:

ospf hellos

Routers R1 and R2 are directly connected. After OSFP is empowered the two routers send Hellos to one another to set up a neighbor relationship. You can check that the neighbor relationship has for sure been built up by composing the show IP OSPF neighbor’s order.




In the example above, you can see that the router-id of R2 is Each OSPF router is doled out a router ID. A router ID is dictated by utilizing one of the following:

 1. utilizing the router-id order under the OSPF procedure.                                                                   

 2. utilizing the most elevated IP address of the router’s loopback interfaces.
 3. utilizing the most elevated IP address of the router’s physical interfaces.

The accompanying fields in the Hello parcels must be the equivalent on the two routers with the goal for routers to wind up neighbors:

  • subnet
  • region id
  • hi and dead interim clocks
  • verification
  • region stub signal
  • MTU

As a matter of course, OSPF sends hi bundles every 10 seconds on an Ethernet arrange (Hello interim). A dead clock is four times the estimation of the welcome interim, so if routers on an Ethernet arrange doesn’t get something like one Hello parcel from an OSFP neighbor for 40 seconds, the routers proclaim that neighbor to be down.

OSPF neighbor states

Prior to setting up a neighbor relationship, OSPF routers need to experience a few state changes. These states are clarified beneath.

1. Init state – a router has gotten a Hello message from the other OSFP router
2. 2-way state – the neighbor has gotten the Hello message and answered with his very own Hello message
3. Exstart state – the start of the LSDB trade between the two routers. Routers are beginning to trade connect state data.
4. Trade state – DBD (Database Descriptor) bundles are traded. DBDs contain LSAs headers. Routers will utilize this data to perceive what LSAs should be traded.

5. Loading state – one neighbor sends LSRs (Link State Requests) for each network it doesn’t think about. The other neighbor answers with the LSU’s (Link State Updates) which contain data about asked for networks. After all, the asked for data have been gotten, other neighbor experiences a similar procedure
6. Full state – the two routers have the synchronized database and are completely adjoining with one another.


OSPF areas

OSPF utilizes the idea of areas. A region is a legitimate gathering of touching networks and routers. All routers in a similar zone have a similar topology table, yet they don’t think about routers in alternate territories. The primary advantages of making zones are that the measure of the topology and the routing table on a router is reduced, less time is required to run the SFP algorithm and routing updates are also reduced.

Every region in the OSPF organize needs to interface with the spine region (zone 0). All router inside a region must have a similar territory ID to end up OSPF neighbors. A router that has interfaces in excess of one (area 0 and area 1, for example) is called Area Border Router (ABR). A router that interfaces an OSPF system to other directing areas (EIGRP network, for example) is called the Autonomous System Border Router (ASBR).


In OSPF, manual course rundown is possible just on ABRs and ASBRs.

To more readily comprehend the idea of territories, think about the accompanying model.

ospf areas

All routers are running OSPF. Routers R1 and R2 are inside the backbone area (zone 0). Router R3 is an ABR, on the grounds that it has interfaces in two territories, specifically zone 0 and territory 1. Router R4 and R5 are inside area 1. Router R6 is an ASBR, on the grounds that it associates OSFP system to another steering space (an EIGRP area for this case). On the off chance that the R1’s straightforwardly associated subnet falls flat, router R1 sends the directing refresh just to R2 and R3, in light of the fact that all routing updates all confined inside the zone.


The job of an ABR is to promote deliver synopses to neighboring territories. The job of an ASBR is to associate an OSPF routing area to another outer system (e.g. Web, EIGRP organize… ).


The LSAs (Link-State Advertisements) are used by OSPF routers to trade topology data. Each LSA contains routing and topology data to portray a piece of an OSPF network. At the point when two neighbors choose to trade routes, they send each other a rundown of all LSAa in their individual topology database. Each router at that point checks its topology database and sends a Link State Request (LSR) message asking for all LSAs not found in its topology table. Another router reacts with the Link State Update (LSU) that contains all LSAs asked for by the other neighbor.

The idea is clarified in the following example:

ospf lsa lsr lsu

After configuring OSPF on the two routers, routers exchange LSAs to depict their particular topology database. Router R1 sends an LSA header for its straightforwardly associated system Router R2 checks its topology database and verifies that it doesn’t have data about that network. Router R2 at that point sends Link State Request message asking for additional data about that network. Router R1 reacts with Link State Update which contains data about subnet (next jump address, cost… ).