What is the function of auto-cost reference bandwidth command?

Use this command to configure the cost of an interface that a device advertises to it’s OSPF neighbors. OSPF calculates the cost of a route as the ratio of the reference bandwidth to the bandwidth of the egress interface. An increase in the reference bandwidth results in an increased cost.

What is reference bandwidth?

The reference bandwidth is a value in Mbps that we can set ourselves. By default this is 100Mbps on Cisco IOS routers. The interface bandwidth is something we can lookup.

How does OSPF check reference bandwidth?

The default OSPF metric is calculated by dividing the reference bandwidth by the bandwidth of the interface. The default value of the reference bandwidth is 10^8 or 100,000,000. Using this formula, the OSPF cost for a T1 is 65 (100,000,000/1,544,000).

How do I show OSPF cost?

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.

How do you set auto cost reference bandwidth?

To adjust the reference bandwidth, use the auto-cost reference-bandwidth Mb/s router configuration command. This command must be configured on every router in the OSPF domain. Notice that the value is expressed in Mb/s; therefore, to adjust the costs for: Gigabit Ethernet – auto-cost reference-bandwidth 1000.

How do I select a designated router in OSPF?

Two rules are used to elect a DR and BDR:

1. router with the highest OSPF priority will become a DR. By default, all routers have a priority of 1.
2. if there is a tie, a router with the highest router ID wins the election. The router with the second highest OSPF priority or router ID will become a BDR.

What does the bandwidth command do in Cisco?

The bandwidth command sets an informational parameter to communicate the current bandwidth to the higher-level protocols. IGRP uses the minimum path bandwidth to determine a routing metric. The TCP protocol adjusts initial retransmission parameters based on the apparent bandwidth of the outgoing interface.

How do I change bandwidth in OSPF?

What is the OSPF cost for a T 1 link bandwidth 1.544 Mbps given the reference bandwidth of 100Mbps?

We divide 100 by 100 and that gives us a cost of 1. On a serial interface, like a T1 in that example here, the bandwidth is 1.544 Mbps. That will default to a cost of 64 because a hundred divided by 1.544 is 64.

How do I change the OSPF cost on a Cisco router?

Cisco routers have three methods to change the OSPF interface cost:

1. By directly using the interface command ‘ip ospf cost <1-65535>’ Router#conf t Router(config)#int gi0/0/0 Router(config-if)#ip ospf cost <1-65535>
2. Changing the ‘interface bandwidth’ setting (in kilobits), which changes the calculated value.

How do I advertise a loopback in OSPF?

To advertise the route of a loopback interface with the configured subnet mask, you can use the following method.

1. Configure the network type as POINT_TO_POINT.
2. Summarize the routes of the loopback interface.
3. Redistribute the route of the loopback interface to OSPF.

How do I change the reference bandwidth in OSPF?

How does OSPF choose DR and BDR?

How do I check bandwidth usage?

1. Track Bandwidth Usage via Your Router. The best place to start figuring out what is consuming your bandwidth is your router.
2. Check Bandwidth Usage With Capsa.
3. Scan Your System for Malware.
4. Use Netstat to Uncover Network Issues.
5. Check Network Activity With Windows Resource Monitor.

How do you set auto-cost reference bandwidth?

Why would you want to change the OSPF default reference bandwidth?

By default, the reference bandwidth is 100 Mbps. However, if you have faster links in your network, such as gigabit Ethernet or OC-3 connections, OSPF can’t give these links a better cost than 1. So you should set the reference bandwidth to at least as high as the fastest link in your network.

How do I change my reference bandwidth?

How do I advertise an interface in OSPF?

To enable OSPF on a Cisco router and advertise interfaces, the following tasks are required:

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.

What is auto cost reference bandwidth in OSPF?

OSPF Cost. OSPF uses a reference bandwidth of 100 Mb/s for any links that are equal to or faster than a fast Ethernet connection. Therefore, the cost assigned to a fast Ethernet interface with an interface bandwidth of 100 Mb/s would equal 1. Cost = 100,000,000 bps / 100,000,000 = 1.

What is the cost of the auto-cost reference-bandwidth commands?

cost= 100000000/bandwith in bps The auto-cost reference-bandwidth commands changes the first value in the above formula, but uses Mbits per second as the argument, so the formula will now be cost= 52428800000/bandwith in bps

What is OSPF auto-cost reference bandwidth?

Auto-cost reference bandwidth is used to modify the OSPF metric. The OSPF metric is cost. Cost is calculated by dividing 100 by the bandwidth in Mbps. Cost = 100/ (bw in Mbps) Any result less than 0 will use a cost of 1. Default OSPF costs using the default reference bandwidth. Interface Bandwidth. Cost. 10 Gigabit Ethernet 10Gbps.

What happens if I change the reference bandwidth?

Hello Alman, if you change the reference bandwidth you need to do it on all your routers or possible routing problems can occur. With your change for example the OSPF cost of a GE interface becomes 50 (from 1 using the original reference BW) so it is important to be consistent in this parameter. Hope to help Giuseppe 3 Helpful Reply amacdos

What are the pros and cons of the OSPF cost command?

The con is that it affects all of your interface costs. You could just change your costs manually with the ip ospf cost command on the interface as well and that will give you more granularity. If you use the auto-cost command, it’s recommended that you change it everywhere. 03-17-2012 05:37 PM 03-17-2012 05:37 PM