We're continuing on with our discussion of Basic OSPF and in this video we're going to discuss the OSPF Cost. Now as you'll recall from EIGRP and from discussions of other routing protocols if you went the CCNA route for this that every routing protocol has it's own metric for RIP it's hop count, for EIGRP it's that big long mathematical formula that incorporates all the K values and the bandwidth and all that nice stuff. For OSPF it is the Network Cost and luckily the Network Cost is pretty simple to calculate for OSPF. You don't have to have a degree in mathematics like you do with EIGRP. You just have to know what the reference bandwidth and the actual bandwidth of your interfaces are and then you can determine the cost to any network. So let's look back at our sample network here, our lab network here. We're going to concentrate on the Corp router. Now obviously these are all Ethernet connections, it's going through this little switch here. These are serial connections, 1.544 megabyte, it's a full T1. I haven't gone and, and sliced and diced those T1s up so obviously the cost to get to these networks are higher than the cost to get to these networks, just simple logic would tell you 100 megabyte is faster or 10 megabyte is even faster than 1.544 and if we look in the routing table on the Corporate router, I'll filter this just for OSPF routes actually. The OSPF Cost that's assigned to these networks bears this out. The 10.11 and the 10.12 networks which are these two networks over in sales and engineering, they both have a cost of two. That is the second number here in this, this bracket here. The first is the Administrative Distance and we'll talk more about the AD when we discuss route redistribution elsewhere in the course. But obviously those directly connected or you know, pretty much directly connected networks have a cost of two or as the networks that across serial links the 10.1 and the 10.2 networks that you see up there, have a cost of 65. Alternately let's look at this from Tennessee's perspective, if we go over to Tennessee and we do a Show IP Route OSPF on that router, we'll notice that we have a few routes, the 10.10, the 10.11 networks have costs of 65. This 10.1 networks over here in Georgia have a cost of 129 because obviously to get from Tennessee to Georgia in our configuration, you have to truck the traffic down here to the corporate office and then back out to Georgia up here to get to those networks, so you got to cross two of those serial links. So now that we've discussed how the cost is used, how is it calculated. Well, quite simply the OSPF Cost is the reference bandwidth divided by the interface bandwidth and all the calculations are in kilobytes. The reference bandwidth is 100 megabyte by default. Now let me pull this Notepad over here, just so we can keep track of this. So there's the OSPF Cost Formula. So let's figure out the cost for a serial link. This serial link is 1.544 megabyte, so if we take 100 megabyte which is 100,000 kilobytes and I know the math nerds are saying, no it's actually 102,400 kilobytes, well, it, it's, this is the calculation that OSPF uses, I'm more of a purist and, and think that myself as well, but this is how the calculations done. In this case, 100,000 kilobytes divided by 1.544 that nets us 64.7 which if you round up it's 65. And if we look in the route table for Tennessee, we see that the 10.20 network is indeed a cost of 65. Now that's simply because you only have to cross this one serial link and then you're on this network which you don't have to actually cross this link to get to that network, you're on it at this point. Now obviously the link to these 10.11 and 10.12 networks are higher than 65, they are 66 and that's simply because you have to cross this network which is a cost of 65 and this network which is a cost of one to get to this .11, .12 network on this end. So quite simply you can figure out the cost to get to any network in your OSPF topology by adding together the interface costs for all the networks you cross. If you cross this network and this network and let's say there are four or five more routers down here, you add them up and you get the cost to get there from Tennessee or from Georgia or from engineering. Now I know what some of you are probably saying, there's probably up there, raising their hands, saying well what about if I have networks that faster than a 100 megabyte, what if I have gigabyte? Which you know, the company I work at, we have gigabyte all the way to desktop, but if I have 10 gigabyte or 100 gigabyte or larger than 100 megabyte interfaces scattered throughout my network? Well the short answer is, anything above 100 megabyte has an interface cost of one. If you had three interfaces of this router, you had two 100 megabyte and a gigabyte, they would each have an interface cost of one to get across that network. Now there are two ways you can tweak this behavior and we'll do both of them in our advanced lab in the other portions of this course. You can go into a specific interface and say the cost to cross this interface is not one, it's twenty or thirty or three billion or whatever number you want to plug in there. Alternatively, you can change the reference bandwidth. If you'll recall our formula is reference bandwidth divided by interface bandwidth. If we change the reference bandwidth to be one gigabyte or ten gigabyte then the cost for lower speed interfaces, 100 megabyte, ten megabyte et cetera, goes up, because this formula now yields a higher number. And as we put this entire course together, you'll play with the OSPF Cost, you'll play with the Administrative Distances, when we put together some of the more advanced labs for OSPF and for route redistribution but that pretty much concludes our discussion of the OSPF Cost Metric.
|Course:||Implementing Cisco IP Routing (642-902 ROUTE)|
|Duration:||10 hrs / 105 lessons|
|Captions:||Available on CD and Online University|
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