Implementing Cisco IP Routing (642-902 ROUTE) Tutorials
BGP / Lab - Route Selection in BGP pt. 3
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Subtitles of the Movie (Implementing Cisco IP Routing (642-902 ROUTE) / BGP / Lab - Route Selection in BGP pt. 3)
So we're continuing our discussion of the Route Selection Process in BGP and we've determined that we want the path to this 151.1.1.0 network to go out a different path. We want it to go out 50.1.1.1 or basically go out the Access 1 Router from our Internal Network here, rather than going out Access 2. So the question is, how do we do this? Well there's a couple of different ways we can do it and we'll step through each of them and then I'll show you the way that I would do it, if I were presented with this in an actual Production Network. If we'll look back at our list for our BGP Route Decisions, we notice that other than networks where the next hop is accessible, we'll just really ignore that one for, for most of these Routing Decisions because obviously we're not going to try to advertise networks that we can't actually get to. Prefer the path with the highest Weight. The first thing that we can modify or the first thing that we can influence is the Weight. Now the Weight can be applied on a per neighbor or a per route basis for the CCNP exam, all you'd really have to know how to do is apply the Weight to a specific neighbor. Now the higher Weight wins so if you apply a Weight to a neighbor using the Neighbor Commands in BGP then that means any routes that come from that neighbor will be preferred over all of the other routes. Which means if we do this, then Access 1 will send all of it's traffic out of it's ISP 1 Router. Now the Weight is local as I've mentioned before, so that would not effect other Routing Decisions on Access 2. Access 2 would still continue to send part of it's routes out of this ISP 2 Router and then send some of it's routes over to Access 1 because from Access 2's perspective, Access 1 has the better routes for certain networks. And that's a good enough solution. It's not the most elegant solution because that means Access 1 will still send all of it's traffic out this way even for these routes over here that it's obviously closer for it to go out Access 2 to get to this 150.10 over here. However, we'll show you how the command works just because you'll probably need to know it for the exam. So if we go into Config Mode and we do Router BGP, Router BGP 12 and we do Neighbor and then put in the neighbors IP, in this case, 50.1.1.1. Let's verify that, that is actually it. 50.1.1.1 that's ISP 1. If we hit the Question Mark here, the command we're looking for is the last one in the list down here. It is the Weight. Set the Default Weight for the routes from this neighbor. Now if we scroll back up in the window we see that the Default Weight is zero. Now the Default Weight for all routes is zero because obviously it's not something that you normally would have to set. So any number above zero will make Access 1 prefer that neighbor over all others for all the routes. So we could set it to, in this case, we'll do Weight and we could set it to any number between zero and 65,535. We could set it as low as two, and that would make it would prefer all the routes going out that way. So that's how you would do that. Let's go ahead and Enter that there and let's do Show IP BGP and you'll notice that the Weights are not actually taking effect and the reason for that is, anytime you modify a parameter that effects routing in anyway, you have to basically reset the BGP process on this Router. So we'll type Clear IP BGP star. And that basically forces the neighbors down and then waits for them to re-establish which we'll pause the video, we'll wait for them to re-establish and then we'll look at the BGP Table again and see how the Weight gets stuck into the BGP Table and how that effects the Routing Table and the Routing Decisions from Access 1. So now our neighbors are back up so we'll do Show IP BGP, we'll notice that now we have a Weight of two for all of the routes that are coming in from 10, 50.1.1.1 and for all of the ones where the Weight is set to two, it is preferring those routes, even though it may not be the most efficient. Like, for example, this route right here, the AS Path is longer but because the AS Path is of a lower precedence than the Weight, then that route that's longer wins. If we now look at the Routing Table, we now see that every BGP Route that we know about with the exception of the 10.20 route for the other Ethernet Link on the other side of our little AS 12 here, now goes out 50.1.1.1 even if it is not the most efficient route, it still sends it out that way because of the Weight. This is obviously not the most elegant solution for this because again, since this is a Local Parameter it does not effect the Routing Decisions on Access 2. If we wanted to effect the Routing Decisions on Access 2 we would have to go down one in the Routing Tree and set the Local Preference. Now the Local Preference is very similar to the Weight, it stays within the Autonomous System so it would stay inside Autonomous System 12 but it would tell all the other Routers in Autonomous System 12, hey for these routes or for this neighbor, I want you to send all of your traffic my direction. Now if we were to just globally set the Local Preference, then the net effect of setting it on Access 1 or setting the Weight on Access 1 would be exactly the same. All of the routes, in this case, all of the routes for everything on our AS 12 network would go out Access 1. Again, not the most elegant solution since according to our little set up here, both of these Access Routers connect to different ISPs so let's look at the most effective solution that uses a combination of these two Attributes or uses actually the local Preference Attribute plus a Route Map that we learned earlier in the course. We'll do that 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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