Implementing Cisco IP Routing (642-902 ROUTE) Tutorials
Basic OSPF / OSPF DR and BDR Roles pt. 1
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Subtitles of the Movie (Implementing Cisco IP Routing (642-902 ROUTE) / Basic OSPF / OSPF DR and BDR Roles pt. 1)
In this video, we're going to discuss a little bit about the OSPF Designated Router and Backup Designated Router Election Process. Now we've touched a little bit on the DR and the BDR elsewhere in the course when we built our little lab a few videos ago and we just turned OSPF on and it just all kind of worked for the most part. However going forward as we build more complex networks with OSPF and as we discuss other network types, you will need to know what the DR and the BDR actually are, what they do, how they function, how the election happens and all that nice stuff. Also if you plan on taking the exam, which I kind of consider the, the whole point of this course, you'll need to know this as well for the test. Quite simply the Designated Router is used on multi-access networks to control routing updates. We'll see exactly how that is here in the next slide or in a couple of slides, but that's it in a nutshell. You have to have a DR on multi-access networks, multi-access networks are Frame Relay multi access Ethernet, networks like that and we'll talk about OSPF network types when we talk about advanced OSPF elsewhere in the course. The Designated Router role is per interface, so don't confuse the designated router with like the ABR or the ASBR or any other router types like that. For example, you could have a single router that has the designated router role on one interface and a BDR on another and it's non designated on yet another interface and again we'll look at this, when we talk about the advanced lab, but just remember the DR role is not specific to the entire OSPF process on a router as a whole, it, it can vary again from interface to interface because there's certain OSPF interface types and certain OSPF network types that don't even use a DR or a BDR. Now for the most part the Designated Router Election Process is automatic although you can influence the Election Process some ways, you can't just go into the router and say, you'll always be the DR and you'll always be the BDR but you can kind of stuff the ballot box if you will, when it goes to elect it's DR and BDR. So how is the DR, the Designated Router elected on the network? Well, quite simply it's Router Priority. You'll recall that Router Priority is one of those fields that's sent in the Hello packets, the Router Priority again doesn't have to match in order for the neighbor relationship to establish but the other neighbors have to know what the Router Priority is. Basically, the router with the highest Router Priority becomes a Designated Router. The Router Priority can be any number from zero to 255, the highest priority router becomes the Designated Router. The second highest priority router becomes the Backup Designated Router. The default priority is one so unless you go in and modify the priority of a router, then by default the priority just doesn't come into play. We'll talk about that here in just a second. You can however, tell a router, you will never become a Designated Router or a Backup Designated Router and you can set a router to priority zero and you'll say that, you'll never be a DR or a BDR and honestly, the DR and BDR Election Process, it's, it's pretty much automatic. You know, you set it up on the network, it just kind of happens, most network engineers don't give it a second thought because you know, unless there's a really specific need for a really specific router to become the Designated Router, which I've never seen on any of the OSPF networks I've worked on, then you just kind of let the routers figure it out amongst themselves. So what happens if the priorities are equals? Again, the default priority is one on every router in your OSPF area, so what happens if you don't change that? In that case, the router with the highest Router ID becomes the Designated Router and the second highest Router ID becomes the Backup Designated Router. You'll remember when we talked about the Router ID and how it can change based upon the presence or absence of certain interfaces. This is one of the ways in which the Neighbor ID or the Router ID is important. So we've talked about basically the, the DR and BDR Election, well what is the Backup Designated Router? The Backup Designated Router is just kind of fail over. It sits there and watches the Designated Router and says are you, are you still there? Are you still alive? Are you solid? Hey he failed, I'm the DR now. And since the Primary Designated Router failed or the real Designated Router failed, the BDR is promoted to the Designated Router and another election is forced to figure out who is now the BDR and I'm not going to elect a new Designated Router because the BDR becomes the Designated Router, but they're, they're fighting over vice presidency at this point. Now something to remember is that if you happen to bring another router online that has a higher priority or a higher Router ID than the current Designated Router, it does not automatically force an election, you have to have some type of failure, you have to the DR or the BDR fail and another election to be forced before all the other routers will agree, well yeah, you've got the higher priority, you need to be the Designated Router or the Backup Designated Router. So we'll see how the DR and the BDR helps a multi-access network manage it's traffic and manage it's routing updates in the next video.
Tutorial Information
| Course: | Implementing Cisco IP Routing (642-902 ROUTE) |
| Author: | Greg Dickinson |
| SKU: | 34291 |
| ISBN: | 978-1-61866-028-2 |
| Release Date: | 2011-12-28 |
| Duration: | 10 hrs / 105 lessons |
| Work Files: |
Yes |
| Captions: | No |
| Compatibility: |
Vista/XP/2000, OS X, Linux QuickTime 7, Flash 8 |
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