So now we're going to configure BGP over a Multi-link connection. You'll notice between Access 2 and ISP 2 we have two T1s and in the real world this might be for Redundancy or added bandwidth or something along those lines. For our purposes, it's just so we can demonstrate how this is set up. So if we look here on Access 2, we show that we have two IP addresses going to this ISP 2 Router. If I do Show CDP Neighbor, you notice that we have ISP 2 Router showing across both Serial 1.1 and Serial 1.0. So the question is, what IP address do you put in the Neighbor Statement? I mean we could set up Neighbor Relationships 188.8.131.52 and 184.108.40.206 but that would mean we'd have twice as many routes in the BGP Routing Table. You'd increase the processing and memory requirements by double just because you have two neighbors as far as the BGP process is concerned. BGP doesn't realize that, that neighbor is really the same Router. So setting this up properly requires a little bit of work on both ends. You have to work with the ISP and they have to work with you obviously to get this set up in the best way possible. Now the first thing we're going to do is set up a Loopback address, another Loopback address on the ISP 2 Router. And we'll give this a bogus address like 220.127.116.11 so we'll do Config T. We'll do Interface Loopback 1. We'll do IP address 18.104.22.168 with a Host Network Mask 255.255.255.255. That way we don't waste the entire 1. network by having bound to this Loopback address. We'll also do the same thing on Access 2. Obviously we'll use a different IP address, but we'll go over here and Config T. Interface Loopback 1, we'll give it an IP address of 22.214.171.124 with a Host Network Mask. And so now if we Exit out, we Show IP Interface Brief, we see now that we have a Loopback address with that address assigned or a Loopback Interface with that address assigned and so now we just have to make sure that we can Ping both of these addresses from each of the Routers. So what we'll have to do is, we'll have to set up a Static Route that says to get to the 126.96.36.199 network address, you go down one of these two links and obviously you'll remember, if we have two Static Routes to the same network going over two different Serial Links that have the same Bandwidth, that the Router will Load Balance across both of these T1s. So go into Config T, we'll do IP Route 188.8.131.52. I think we have to put the Net Mask in here, yes, 255.255.255.255 and the next hop will be 184.108.40.206 and we'll just hit Up Arrow and put 1.1.5 as well. So now in the Access 2 Router if we do Show IP Route we have a Static Route to the 220.127.116.11 host via both of those and both of those are actually installed in the Routing Table because again, both of these T1s are equal cost. So we do the same thing on ISP 2. We'll do IP Route 18.104.22.168 255.255.255.255 and it's next hop will be 22.214.171.124 and 1.1.6. So now if we do Show IP Route on ISP 2 we have all of our connected networks as well as the Static Route to 126.96.36.199. So now if we Ping 188.8.131.52 from this guy, we get Pings back and the same thing from Access 2 if we Ping 184.108.40.206 we get Pings back. So now all that's left to do is set up the BGP Neighbors pointing at those Loopback addresses. However when we set up this neighbor, we have to tell both the Routers that your next neighbor is more than one hop away. Now by default, when you set up EBGP Peers, the EBGP Peers have to be no more than one hop away from one another. Obviously we saw that is not the case with IBGP as Access 1 and Access 2 are two hops away from one another yet they still were able to form an IBGP Neighbor Relationship because this Multihop doesn't apply in IBGP. It's one of the few differences between EBGP and IBGP. However if you've got BGP Peers in different AS's they cannot be more than one hop away. Now in this case, you might be looking at the network diagram and saying but they're not more than hop away, they're directly connected. Well technically the Loopback Interface is seen as another hop so you send out the packet from Access 2 to ISP 2. ISP 2 says well I have to route it to this Loopback Interface where the BGP process is running and the BGP process is expecting to see the Neighbor Relationship form. However since that is seen as an extra hop, the TTL and the BGP packet decrements to zero since it defaults to one and therefore the Neighbor Relationship never sets up. Never fear, setting BGP Multihop is pretty simple it's just another argument on the Neighbor Statement. We'll finish this set up in the next video.
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