## How OSPF cost is calculated?

The Cost formula is reference bandwidth divided by interface bandwidth. The default reference bandwidth of 100 Mbps is used for OSPF cost calculation. For example, if we have an Ethernet interface (10 Mbps), the OSPF path cost value is 100 Mbps / 10 Mbps = 10.

## What is the default cost of OSPF?

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.

How many tables are there in OSPF?

three separate
The OSPF process builds and maintains three separate tables: • A neighbor table – contains a list of all neighboring routers. A topology table – contains a list of all possible routes to all known networks within an area. A routing table – contains the best route for each known network.

### What is OSPF cost preferred?

OSPF prefers lowest cost path to the destination. So if you have 2 routers & if your preference is to reach R1 than R2, then manually assign cost on interface connecting to R1 some what lower than R2.

### How OSPF calculate the best route?

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.

How does OSPF increase cost?

Two methods are available to change the OSPF cost. One method is to run the ospf cost command to directly change the interface cost, and the other is to change the bandwidth reference value to indirectly change the interface cost.

## How do I add cost in OSPF?

Enter configuration commands, one per line. End with CNTL/Z. The other way of affecting the cost of an interface is through the “ip ospf cost” command on the interface itself, this will override the calculated cost. You can also set the cost per neighbor using the “neighbor (neighbor-ip) cost value” command.

## How many routes can OSPF handle?

This is due to the max number of fragments ( 256 ) with each containing not more than 121 routes.? Do the following commands help to change this number? Has anyone seen any such limitation for OSPF?

What is neighbor table in OSPF?

The neighbor relationship table is called an adjacency database in OSPF. If OSPF is configured correctly, it forms neighbor relationships only with directly connected routers. These routers must be in the same area as the interface to form a neighbor relationship. An interface can only belong to a single area.

### How does OSPF determine cost of best path?

OSPF uses a cost metric that represents the status of the link and the bandwidth of the interface in an algorithm to determine the best route to a destination. The algorithm used is called the SPF (shortest path first) or Dijkstra algorithm.

### Does OSPF use hop count?

OSPF routing protocol has complete knowledge of network topology, allowing routers to calculate routes based on incoming requests. OSPF protocol has no limitations in hop count, unlike RIP protocol that has only 15 hops at most.

How does OSPF determine cost of interface?

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.

## What is the area 0 in OSPF?

Backbone Area This is the central area in your OSPF routing domain which all areas must connect to. This area is labeled area 0 (or the longhand version 0.0. 0.0). When traffic needs to pass from one area to another, it must traverse the backbone.

## What are 7 possible states that OSPF can be in?

Following are the OSPF States we’ll be examining:

• Down state.
• Attempt state.
• Init state.
• 2-Way state.
• Exstart state.
• Exchange state.
• Full state.

Why is OSPF faster than RIP?

OSPF protocol has no limitations in hop count, unlike RIP protocol that has only 15 hops at most. So OSPF converges faster than RIP and has better load balancing. OSPF multicasts link-state updates and sends the updates only when there is a change in the network.

### Why is OSPF not scalable?

In theory, OSPF topology is limited by the number of links that can be advertised in the Router LSA as each router gets only one Router LSA and it cant be bigger than 64K which is the biggest an IP packet can be. The same constraint applies to the Network LSA also.

### What is 2-way state in OSPF?

2-Way State This state describes the Bi-Directional communication state, Bi- Directional means that each router has received the other’s Hello packet and that each router can see its own Router ID included within the Hello packet’s neighbor field.

Why is OSPF the best?

Pros: OSPF routing protocol has complete knowledge of network topology, allowing routers to calculate routes based on incoming requests. OSPF protocol has no limitations in hop count, unlike RIP protocol that has only 15 hops at most. So OSPF converges faster than RIP and has better load balancing.

## What is the maximum hop count for OSPF?

OSPF protocol has no limitations in hop count, unlike RIP protocol that has only 15 hops at most.

## What is OSPF cost?

OSPF cost is its metric (the unit used by a router to make routing decisions). OSPF executes a process using an algorithm developed by Edsger Dijkstra to find the shortest route to the advertising node. The router will then use this output as a candidate path for its routing table.

What is the difference between OSPF algorithm and cumulative cost?

SPF algorithm calculates all possible routes from source router to destination network. Cumulative cost is the sum of the all costs of the outgoing OSPF interfaces in the path. While calculating cumulative cost, OSPF consider only outgoing interfaces in path.

### How can OSPF calculate the cost of an interface based on bandwidth?

So that OSPF may be able to calculate the cost of an interface based on the bandwidth of the interface with great accuracy we will need to change the value of the auto-cost reference-bandwidth value from 100,000,000 to a greater value. To change the reference bandwidth , we can use the following commands:

### How do I check the BW used for OSPF costing?

The show interface command verifies the BW used for OSPF costing. You can use the show ip protocols command to see the reference-bandwidth, which is the value of the numerator of the OSPF cost formula. What if the router ID of the OSPF router is duplicated?