Implementing Cisco IP Routing (642-902 ROUTE) Tutorials
Basic OSPF / OSPF DR and BDR Roles pt. 2
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Subtitles of the Movie (Implementing Cisco IP Routing (642-902 ROUTE) / Basic OSPF / OSPF DR and BDR Roles pt. 2)
So we're discussing the Designated Router and the Backup Designated Router Election Process. We've talked about how they're elected and what influences that election and how you can stuff the ballot box. What good is a DR and a BDR? Yeah you know, you've got to have them on certain network types, what do they actually do? How do they help us? Well let's talk about that. So let's assume we have a Network Topology that's like this. You've got these routers here that are connected with serial links and then you've got one, two, three, four, five, six routers here on the same Ethernet segment and let's assume this is like a corporate office or, or something like that. So you've got all of these routers that are connecting DMZ's and branches and everything like that, all on this one segment. So we will assume that the serial link to this network down here goes away, this, this network goes away, this router after the Dead Timer expires sees, well this neighbor's down because this interface is down, well hey, a Link State Event has happened, I need to tell everybody about it. Now if we didn't have a Designated Router or a Backup Designated Router, this router would send Link State Updates to all of it's neighbors. We're assuming that it's neighbors with all of these routers here and then each of these routers will accept the update, say well I need to tell all of my neighbors and so it will proceed to tell all of it's neighbors except the one the update came from of course. So this router would tell all of these routers, this router and I didn't draw all the arrows on here because it would very quickly become confusing and it took me about an hour just to get this slide where I could, I could manipulate it. But as you can see, a network down event would just cause this flurry of traffic on this Ethernet segment as this router tells this router, and it tells here and this guy tells him and he tells that guy and this guy. You can see all this traffic goes back and forth and just drives you nuts, you know, and one, and one of these links goes down and all of a sudden your cores running at 80 percent interface utilization for a couple of minutes while OSPF figures everything out. That is not optimal network design obviously. So let's throw all that away. In a Designated Router and a Backup Designated Router scenario, again this network down here has failed, so instead of telling everybody, this router here, just tells it's Designated Router and Backup Designated Router. It says hey guys, this networks down, thought you all might want to know. Once they've received this update, the DR and the BDR acknowledge the update, they incorporate into their LSA tables and then those two routers tell all of the other routers on the network, hey this Link State Change has happened. So this router gets told by each of these routers, this one gets told by each of them, this one gets told by each. Obviously this router originated the update, so these two routers don't tell this guy, hey your network failed, because you know, doh, of course it failed, I'm the one that told you. And once the DR and BDR tell all of the neighbors on the segment, that's it. These routers don't broadcast it out to everybody else because everybody's looking to these two routers to give them their Link State Updates. Now obviously you've increased your convergence time a little bit, but not by any measurable about. Not by any, any amount that makes any, any difference. And you just completely reduced the traffic load on this Ethernet segment down to practically nothing. So as you can see DRs and BDRs really help in multi-access segments and you'll really see how that happens and how that helps when we talk about some of the advanced network scenarios elsewhere in the course, when we talk about the Frame Relay Multi-Access and so on and so forth. But for the time being that concludes our discussion about the DR and the BDR Election Process.
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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