Implementing Cisco IP Routing (642-902 ROUTE) Tutorials
BGP / Lab - Route Selection in BGP pt. 2
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Subtitles of the Movie (Implementing Cisco IP Routing (642-902 ROUTE) / BGP / Lab - Route Selection in BGP pt. 2)
So let's continue troubleshooting the route to this 151 network down here. Now if we go back to the BGP Route Decision tree if you will, let's just step right down the list and let's look at this route and see if we can figure out why it made the decision that it did. First off, discard paths where the next hop is inaccessible. Well if we look over here, we see that the next hop is an accessible Router, we can get to 10.20.2.2. We can also get to 50.1.1.1 so that's not the tie breaker, so it's not going to discard the route because of that. Prefer the path with the highest Weight. Now the Weight is a locally significant variable as I mentioned before and you can set it on that Cisco Router and it never leaves that Cisco Router. It is an optional Non-Transitive Attribute to go back to the actual terminology that's used. So if we look back over here on our lab PC we see that under Weight Column, well all of them have zero Weight so all the routes have zero Weight by default except locally advertised routes. In this case the Weights 32,768 but in this case both of the Weights for both of these routes are zero. So that's not the tie breaker. Prefer the path with the highest Local Preference. If we go back over here to our Router, we'll see that the Local Preference Column is right here, every Router by default has a Local Preference of 100 or to be more accurate every route has a Local Preference of 100. Now we'll be modifying this and in fact modifying the Local Preferences, the solution to the issue that I'm presenting here but for the time being you'll notice that the Local Preference is 100 for the Internal Route here. It's important to note that the Local Preferences only Internal to your Autonomous System, that's why we don't have a Local Preference for routes that are advertised from outside of our Autonomous System, you know, just this route from 50.1.1.1 does not have a Local Preference at all, only the internal BGP route has the Local Preference of 100. Since only one route has the Local Preference set, this isn't a valid comparison since they both have to have a Local Preference in order for that decision tree or that part of the decision tree to make sense, so we'll go onto the next one. Prefer the path that was learned via a Network Command or by Redistribution from an IGP. And basically that means, Preferred Networks that I'm directly connected to. So if we go back to the lab over here, we'll notice that this Router is not connected to this 151 network and, and usually that's not the case. There are some cases where for example, if Access 1 says well I'm learning about the 10.20 network from Access 2. I'm learning about the 10.20.1 network from Access 2, oh wait I see that as I'm directly connected to it, so I'm going to win and if we look in the BGP Table for this 10.20 network, we'll actually see that. In fact, is it on here? Yes, in fact that's exactly it. You'll notice that in this case it picked the Local Router for the 10.20.1 network because it got to this point in the Decision Tree and said, oh well I'm directly connected to that network so obviously I'm going to win every time. Since the 150.1 network is not directly connected we go onto the next tie breaker. Prefer the path with the shortest Autonomous System Path. Now this is where BGP acts like RIP. It looks at the number of hops that the packet has to go through with respect to Autonomous Systems and determines which one of those is the shorter. If we look down at this 151.10 we'll see that this route up here actually has a longer Autonomous System Path, I'm just showing that as an example. In the network that we're looking at, this 151 network, you'll notice that the Autonomous System Path is the same. 127 and 42 versus 50 and then 55. So it's two Autonomous Systems away, no matter which way you go. Obviously this Attribute does not count Routers that are in the same Autonomous System so looking at it from an Autonomous Systems standpoint, to get to this network down here, I can go through these two Autonomous Systems or these two Autonomous Systems and so that's not the tie breaker. So let's go onto the next item in the list. Prefer the path with the lowest Origin Type. Now the Origin Type is here in the BGP Table and you see that it's this last item over here. The Question Mark versus the I. The Question Mark means Incomplete and basically from a BGP perspective, all that means is that route was Redistributed into BGP using a Redistribute OSPF or Redistribute Static or Redistribute Connected Command. IGP means that it was actually learned from a BGP perspective using the Network Command and if you see EGP well then that just means someone's messing with your head, because they're forcing the Origin Code to be EGP, but EGPs not in use anymore. EGP died about fifteen, twenty years ago and was replaced by BGP. Now in the case of this network, this is the tie breaker. IGP is less than EGP. EGP is less than Incomplete. So Incomplete is the highest Origin Code, IGP is the lowest Origin Code, therefore this route right here with the I as the Origin Code wins the tie because the other route is Incomplete and that's exactly as I engineered it. If you were to go look at this Customer 1 Router and look at the BGP Configuration, Show Run Section BGP, you'll see that I did not use the Network Command over here, I used Redistribute Connected and so since we are Redistributing all of the Connected Networks into BGP that means by the time it gets up to Access 1, the Origin Code is unknown. That's why BGP chose this particular route as the best route. It just went down this Decision Tree list here until it hit one that was the tie breaker and in this case, it was number six. Usually it's going to break up here at number five, at least in any scenario you, that you'll run into the CCNA and the CCNP exams. We'll continue with our discussion of Route Selection in BGP 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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