To buy a interface is inversely proportional towards the bandwidth from the interface. So, a greater bandwidth supplies a less expensive and much more overhead and time delays equal a greater cost. Therefore, one hundred-Mb/s Ethernet line includes a greater cost than the usual 1000-Mb/s Ethernet line. The look below illustrates the formula for cost calculation
The need for the reference price is 100Mbps = 100,000,000 bps, Therefore the formula:
The default OSPF cost formula doesn’t differentiate between interfaces with bandwidth quicker than 100 Mbps. The FastEthernet, Gigabit Ethernet, and 10 GigE interfaces share exactly the same cost since the OSPF cost value should be an integer. Therefore, the default reference bandwidth is placed to 100 Mb/s, and all sorts of links which are quicker than Fast Ethernet in addition have a price of 1. The price value for various kinds of interfaces would be the following:
OSPF Builds up Costs
To by a OSPF route may be the accrued value in the source towards the destination network. For instance, in Figure 2, there’s two routes from Router1 LAN to Router2 LAN. The price to achieve the Router2 LAN from Router1 LAN the following:
- Router1 Fast Ethernet / = 1
- Router2 Ethernet = 10
- The total price to achieve 192.168.20.=11
- Router1 Giga Ethernet /1 = 1
- Router3 Serial 1/1/ = 781
- Router2 Ethernet = 10
Total price to achieve 192.168.20.= 792
The routing table of Router1 for network 192.168.20. in Figure 3 confirms the OSPF metric to achieve the Router2 LAN is really a price of 11.
Modifying the Reference Bandwidth
OSPF uses 100 Mbps bandwidth for reference associated with a links which are comparable to or quicker than a quick Ethernet connection. Therefore, the price allotted to a quick Ethernet with regard bandwidth of 100Mbps would comparable to 1.
This calculation works best for fast Ethernet interfaces, So links quicker than 100 Mb/s calculation is difficult since the OSPF metric just uses integers since it’s total price of a hyperlink. When the calculation is under an integer, OSPF protocol models in the value towards the nearest integer.
So, in the OSPF perspective, an interface by having an interface bandwidth of 100 Mb/s, has got the same cost being an interface having a bandwidth of 1Gbps, 10Gbps and 100 Gbps. So, to make the very best path determination the reference bandwidth should be modified to some greater value than 100Mbps to support faster links than 100Mbps.
Altering the Reference Bandwidth
AlTering the reference bandwidth has no effect on the particular bandwidth capacity around the link. It really affects the OSPF metric calculation. We are able to alter the reference bandwidth with router configuration mode <auto-cost reference-bandwidth Mbps>.
The configuration should be same on every router within the OSPF domain. The worth is within Mbps, so, to regulate the expense for GigaEthernet the command would <auto-cost reference-bandwidth 1000> as well as for 10 Gigabit Ethernet <auto-cost reference-bandwidth 10000>.
To cancel the default reference bandwidth, make use of the command <auto-cost reference-bandwidth 100> in router configuration mode. The table in Figure-1 illustrates the OSPF cost when the reference bandwidth is placed to Gigabit Ethernet (1000) and also the table in figure-2 illustrates the OSPF cost when the reference bandwidth is placed to 10 Gigabit Ethernet links. In table-1 the OSPF make smarter choices since it are now able to differentiate between FastEthernet and Giga Ethernet links. In Table-2 it’s also differentiated between 100Mbps, 1G and 10G links.