So we're finally getting routes into BGP and we've noticed that on the Access 2 Router, if we do Show IP BGP that this route it's valid and it's a good route but it's not going to install it in the Routing Table because the next hop is showing 18.104.22.168. Now where is 22.214.171.124? 126.96.36.199 is ISP 1's Interface going into our Access Router or the Interface leaving AS 127 in a logical sense. And this is a Default Behavior of BGP. Basically since the original route advertisement had this guy as the next hop for this network, then as that advertisement passes into IBGP between the Access 1 and the Access 2 Router, it does not modify the Next Hop Attribute and when that happens you end up with something like this, where, hey this is a valid route, I see it, it's internal, but I can't get to the next hop and obviously the router's intelligent enough to figure out, well if I can't get to the next hop, then I don't need to put this in the Routing Table because I won't be able to get there. Now you can fix this very easily by going onto the Access Router and we're going to add a Neighbor Statement or add an argument to the Neighbor Statement for the Access 2 IBGP Peer. So we'll go into Config T, we'll do Router BGP 12. We're going to do Neighbor and the Neighbors IP address. In this case it's 10.20.2.2 so Neighbor 10.20.2.2 and we're going to use the Next Hop Self argument for this particular neighbor. It says disable the next hop calculation for this neighbor, basically it says for all the networks that I advertise into IBGP, I'm going to set myself to be the next hop. So we'll do Next Hop Self and now we sit and we wait for BGP to synchronize that change out. So now we're over on the Access 2 Router after a few seconds and we now see that this network advertisement for the 188.8.131.52 network now has the carrot saying, it's the best route. The reason why is that the next hop is actually reachable from my router, it's saying the next hop is 10.21.2 which is the inside Ethernet Interface for this Access 1 Router. As a result, if we check the Routing Table, we now see that we have a BGP Route for 184.108.40.206 via 10.21.2. So in theory we should now be able to Ping 220.127.116.11, well that didn't work out. Why didn't that work? Well simply put, you can get to the ISP 1 Router, the ISP 1 Router doesn't have a route to get back to Access 2. See I tricked you, I said it would work didn't I? Well the more astute among you probably figured out as soon as I typed the command that it wouldn't. So how do we fix this? Or how we would go about fixing this? We go onto the Access 1 Router and we Redistribute the 10.20 networks that it knows about via OSPF and directly connected also into BGP and therefore it should then send the 10.20 networks over to ISP 1 and so ISP 1 will now have a route to get to Access 2. So let's go through and do that and see that our logic holds out and it should work but should and does is generally two different things, especially when you're talking about Cisco Routers. So go into Access 1, Configure T. We will do Router BGP 12 and now we're going to use the other command, we're going Redistribute routes from OSPF into BGP. So we're going to do Redistribute and of course this looks just like the Redistribute Commands that we saw for EIGRP and OSPF. In this case we're going to Redistribute OSPF Process ID 1 because that happens to be what I have running on Access 1 and we'll use the Keyword Match. We only want to Redistribute Internal Routes. We don't have any External Routes in this OSPF routing domain but it's always a good practice to get into specifying Internal and External, just simply because you probably don't want to Redistribute routes that may have been Redistributed into OSPF, perhaps you're Redistributing from BGP into OSPF on this other router over here. Well, you don't want to Redistribute those same routes back into BGP on this side. You'll end up with one of those huge Routing Loops so we'll say Match Internal and we'll hit Enter. So now we should be Redistributing all our OSPF Routes into BGP so if we do Show IP BGP, we still have to wait for BGP to catch up. So let's pause the video again and wait for BGP to catch up with us. So now that we've waited for BGP to catch up, we go over here to the ISP 1 Router and look, not only are we learning about the 10.20 networks, we're also about learning about all the 192.168 networks because again those routes were learned via OSPF and so they're automatically Redistributed into BGP just by that OSPF Command. Now we could have specified a Route Map to not Redistribute these 192.168 networks and on a production network, you would probably would want to do that. Of course the same argument could be made that any ISP that's worth their salt will be filtering Internal Networks on their external connection to you, so that if you do happen to Redistribute your Internal Networks into BGP, that the ISP will just drop those networks before they can be injected further up the line. But you always want to take the initiative to prevent network issues rather than relying on the ISP to do so. So now we see these routes in the ISP Router. So now in fact, we should actually be able to go back to Access 2 and do Ping 18.104.22.168 and we're still timing out. Well what else could be the problem? Let's investigate that in the next video.
|Course:||Implementing Cisco IP Routing (642-902 ROUTE)|
|Duration:||10 hrs / 105 lessons|
|Captions:||Available on CD and Online University|
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