So we're picking up with our discussion of Modifying the Route Behavior in BGP. Now we're ready to single out just this one network down here and show you how to do it in a very controlled manner, so just this one network gets routed out of Access 1 and all the rest of the networks take the Default Routing Behavior in BGP which is going to be AS Count. So first thing we'll do is we'll set up an Access List that specifies just this network down here. So if we go into Config Mode, go into Config T and we're going to set up a Standard Access List that specifies just this remote WAN like so. And I'm permitting any network that starts with a 18.104.22.168 even though I could be very specific and just put a .2 at the end, I stuck with .255 just to make the Network Command a, a little simpler to see when you're troubleshooting. So now that we've set up an ACL that specifies just that one network, now we're going to set up a Route Map that watches for that network to come in and sets the Local Preference accordingly for that one network. And there is our Route Map set up. We permit 10, we're going to permit anything that matches this Remote WAN Access List. We're going to set the Local Preference to 150 for that one network. The next statement is an Empty Permit Statement to negate the Default Deny that's at the bottom. Now I found out when doing this lab, the hard way or when setting up this lab the hard way, that if you leave off that second command, that Route Map Local Permit 20, if you'll recall from our discussions of Route Maps, if it doesn't match this network here then it just doesn't get advertised and so you'll only advertise this 22.214.171.124 network from this Access 1 Router, everything else will obviously go out Access 2. So now we've got the Access List, we've got the Route Map. Now we have to tell BGP to apply this Route Map to this particular neighbor, in this case, 126.96.36.199. So we will Exit from here, we will go into Router BGP 12 and we will set yet another Neighbor Command, Neighbor 188.8.131.52 Route Map Local Preference which is our Route Map up here and we're going to apply it to the inbound routes from this neighbor that matched this Route Map. Confusing? Well not really, when, when you sit and think about it, we're specifying this neighbor, this Route Map on the inbound routes. Now we'll clear BGP and while we're waiting for BGP to come back up. I'll explain why we're applying it to the inbound routes instead of the outbound routes. So we'll do Clear IP BGP star and now let's go back to our topology, so under normal circumstances I would receive the routes from ISP 1. I would apply them to my BGP Table here with the Default Local Preference of 100. I would then modify the Default Local Preference to 150 and send it out to the Access 2 Router saying that hey I have the Local Preference 150 route for this network. However this guy still thinks that his Local Preference is 100 and so as a result, he's going to say well this guy over here has fewer AS hops or to be more accurate his Origin Code is higher, since it's the same number of AS hops. And so I'm going to prefer him so what happens is, the traffic comes into Access 1 bound for this network. Access 1 says well he's got the better route, so it sends it to over to Access 2. Access 2 says, but no, you've got the Local Preference of 150 according to my Local BGP Table, I'm sending it back to you. Access 1 says, no, no really, you need to take this packet and you need to send it along it's way. Access 2 says but you have the better routes and they just go back and forth like two kids arguing in the back seat until you turn around and threaten to turn the car around if they don't stop arguing. By applying it to the inbound routes, Access 1 applies it to it's BGP Table with the Local Preference of 150. It then advertises that Local Preference of 150 that's in it's Local BGP Table out to Access 2, so both of the Routers agree, Access 1 has the higher Local Preference route and so all the traffic for this 184.108.40.206 network should go out Access 1. At least if I've set this up right. So let's see if I've done it right. Obviously I've talked long enough that BGPs gone down, come back up. You'll notice it's taking anywhere from 30 to 45 seconds so that should give you an idea of about how long it takes BGP to work. So let's do Show IP BGP on this guy. Obviously he's getting Local Preference routes 100 from the other Router, the 10.20.2.2 and for routes that are advertised from him, 220.127.116.11 he sees that as 150 Local Preference routes and so for the network in question, 18.104.22.168, I'll wait until it stops scrolling. 22.214.171.124 even though this particular Router may not have the best path because the Origin Code is unknown, it's still picking that as the best route for this network. If we go over to Access 2, Access 2 should agree. If we do Show IP BGP on this guy we should see the 126.96.36.199 network or the 188.8.131.52 network. The next hop is 10.20.1.2 because he has the Local Preference of 150 even though according to this guy his Origin Code is lower. This guy has the higher Local Preference and if we go back to our graphic that shows us the order that BGP evaluates the Path Attributes, we'll see that Local Preference overrides AS Path. If the tie breaker takes place here at Step 3, Prefer the Path with the highest Local Preference, then it never makes it down to the AS Path or the Original Type which are the two that normally would make the decision most of the time in BGP. So as you can see, it's pretty easy to modify BGPs Default Routing Decisions. Either on a per Router or a per neighbor basis or just for a single network if that's what you so desire. That concludes this part of Modifying the Default Routing behavior.
|Course:||Implementing Cisco IP Routing (642-902 ROUTE)|
|Duration:||10 hrs / 105 lessons|
|Captions:||Available on CD and Online University|
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