OSPF uses SPT tree to calculate the best route for routing table. A SPT tree cannot grow beyond the area. So if a router has interfaces in multiple areas, it needs to build separate tree for each area. SPF algorithm calculates all possible routes from source router to destination network.

Formula for OSPF cost calculation is:

1. Interface Cost= Reference bandwidth/interface bandwidth.
2. Hence By default Interface Cost= 10^8/(interface bandwidth)
3. The “auto-cost reference-bandwidth” command allows you to change the reference bandwidth that OSPF uses to calculate its metrics:
4. Router#configure terminal.

Like EIGRP, OSPF converges quickly. Like EIGRP, OSPF bases its metric by default on link bandwidth, so that OSPF makes a better choice than simply relying on the router hop-count metric used by RIP.

By default, OSPF assigns a default cost metric of 1 to any link faster than 100 Mbps, and a default cost metric of 0 to the loopback interface (lo0). No bandwidth is associated with the loopback interface. This means that all interfaces faster than 100 Mbps have the same default cost metric of 1.

Use the ip ospf cost command to configure the cost metric manually for each interface. This command overrides any settings for the reference bandwidth that you set using the reference-bandwidth command in router configuration mode. This command requires the LAN Base Services license.

If there are multiple routes to a network with the same route type, the OSPF metric calculated as cost based on the bandwidth is used for selecting the best route. The route with the lowest value for cost is chosen as the best route.

Open Shortest Path First (OSPF) is a standards-based routing protocol for IPv4. It is used to advertise paths to a network prefix between routers. OSPF uses costs which are inversely proportional to the bandwidth of the link. As a result, higher bandwidth links are preferred over lower.

The difference between the two metrics is how OSPF calculates the cost of the route. This means that Type 1 external metrics include the external cost to the destination as well as the cost (metric) to reach the AS boundary router. Type 2 external metrics are greater than the cost of any path internal to the AS.

Sets the priority of a router to determine the designated OSPF router for a network. A number between 0 and 255 that specifies the priority of this interface. Each broadcast and Non-Broadcast Multi-Access (NBMA) network has a designated router.

In multi-access networks the router with the highest priority value is chosen as the DR which acts as the central point of LSAs exchange. The priority command is assigned on an interface. Default priority for an OSPF interface is 1. The range is from 0 to 255.

You can use the show ip ospf neighbor command to observe the neighbor data structure. This command displays OSPF-related neighbor information.

To clear redistribution based on the OSPF routing process ID, use the clear ip ospf command in privileged EXEC mode. Syntax Description pid (Optional) Process ID. Reset OSPF process.

Edited by Admin February 16, 2020 at 3:06 AM. After configuring the router-id command, use the clear ip ospf process EXEC command to restart the OSPF. routing process, so the router reselects the new IP address as its router ID.

For both IPv4 and IPv6 networks, you can clear all routes in the routing table by entering the TCP/IP ROUTE command with the CLEAR and the NOW options. The NOW option clears dynamic and static routes (manually configured routes) including those that have active dialogs associated with them.

OSPF area number – Enter the show ip ospf interface interface-name command in order to check. OSPF area type, such as stub or NSSA – Enter the show ip ospf command in order to check.

RIP vs OSPF: What Is the Difference?

Route flapping is caused by pathological conditions (hardware errors, software errors, configuration errors, intermittent errors in communications links, unreliable connections, etc.) within the network which cause certain reachability information to be repeatedly advertised and withdrawn.

The process ID is the ID of the OSPF process to which the interface belongs. The process ID is local to the router, and two OSPF neighboring routers can have different OSPF process IDs.

OSPF Basic Configuration Commands When configuring OSPF, process-id is used as an identifier of the local OSPF process; different OSPF process IDs would be used on the same router if multiple independent OSPF processes were being run on the same device.

Configuring OSPF in a Single Area

1. Use the command router ospf process ID to start OSPF.
2. Use the network command to enable the interfaces.
3. Identify area assignments.
4. (Optional) Assign the router ID. Example 3-1 displays OSPF with a process ID of 1 and places all interfaces configured with an IP address in area 0. The network command network 0.0.

The OSPF process-id is a numeric value local to the router. It does not have to match process-ids on other routers. They are locally significant only, and have no bearing on the structure of any OSPF packet or LSA update. So you can have a separate process-id on every single router in your network if you so desire!

NOTE: the process id must match on all routers in the routing domain for EIGRP to work. This command is used to initiate EIGRP on a router. The process-ID, in EIGRP is a numeric value between 1 and 65536. Dynamic routing protocols work by advertising their directly connected networks.

The OSPF protocol is a link-state routing protocol, which means that the routers exchange topology information with their nearest neighbors. The main advantage of a link state routing protocol like OSPF is that the complete knowledge of topology allows routers to calculate routes that satisfy particular criteria.

Scale: BGP is more flexible and scalable than OSPF and it is also used on a larger network. Preferred path: OSPF is used to determine the fastest route while BGP puts emphasis on determining the best path. Protocol: In OSPF, internet protocol is used. While in BGP, transmission control protocol is used.

OSPF is type of hierarchical network topology or design. OSPF prefers fastest path rather than shortest path. In Open Shortest Path First, internet protocol is used….Related Articles.