Implementing Cisco IP Routing (642-902 ROUTE) Tutorials
BGP / Lab - BGP over Multilink Connections pt. 2
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Subtitles of the Movie (Implementing Cisco IP Routing (642-902 ROUTE) / BGP / Lab - BGP over Multilink Connections pt. 2)
So we're in the process of getting Access 2 and ISP 2 talking to one another. We have set up the Loopback addresses on back Access 2 and ISP 2. They're routing Load Balanced over these two T1s. So now let's go through the configuration that's necessary to make these two Routers talk to one another. Now the configuration is a little tricky but it makes logical sense all the steps you have to go through when it's explained to you as am about to do. So we'll start off on the Access 2 Router. First we'll hit IP BGP and we'll make sure that we're not actually seeing those networks some other way and we're not, we're not seeing the 150.10 11 or 12 networks. We see the 50 networks and if you look at the I, it might look like a one, and it tricked me a little bit when I was first setting this lab up, but we're seeing the 50. We don't see any of the 150 networks because they're not being distributed into BGP. So the first thing we'll do is we'll set up a Neighbor between Access 2 and ISP 2 and we'll set it up to the Loopback that's on ISP 2, that 1.1.1.1 address. So we'll go into Config T, we'll do Router BGP 12. We'll do Neighbor 1.1.1.1, Remote AS will be 50 if I remember right, yes it's AS 50. So Remote AS is 50 and now we have to set two more Neighbor Commands as well. If we just set up Neighbor 2.2.2.2 on the ISP 2 side the Neighbor Relationship would never form and there's two reasons for this. Number one the BGP RFC states that External BGP Peers can be no more than one hop away from each other and you might look at this and say well they are only one hop away. They're directly connected via these two T1s. But remember we're using Loopback Interfaces and so technically the packet is originating on a Loopback Interface that's on Access 2. It goes through Access 2 over to ISP 2 and then out the Loopback Interface on ISP 2. So technically they're two hops away from each other. Unless we configure BGP to allow these Peers to communicate over more than the hop, then the BGP Neighbor Relationship will never form. And we do this using the EBGP Multihop argument on the Neighbor Command. So Neighbor 1.1.1.1 EBGP Multihop and you can set the maximum hop count up to 255 and if you don't specify a number here then it assumes a maximum hop count of 255. That's kind of dangerous in my opinion and the reason why is that say for whatever reason you see this 1.1.1 address coming around this way as well. So you could technically form a Neighbor Relationship going around this way as well as across this T1 connection. Now obviously this Access 2 Router will always prefer this connection rather than going all the way this way, but you are actually doubling the load on the Access 2 Router. So I would say that the EBGP Multihop Command never needs to be set more than about four or five, we'll set it to five in this case. So now we've told Neighbor 1.1.1.1, it's Remote Autonomous System is 50 and you can be up to five hops away and there's one more command that we have to put in the Neighbor Configuration before it will work. And we'll go back to the Neighbor 1.1.1.1 hit the Question Mark and the command we're looking for is the Update Source. Now unless we specify the Update Source this Router's going to say oh well 2, 1.1.1.1 is directly across this T1 link so I'm going to source my updates from this Serial Interface, one or the other, the .6 or the .2. It's going to hit ISP 2, it's going to see the Update coming from not 2.2.2.2 but coming from one of these Serial Interfaces and ISP 2's going to say, well that's not configured as a BGP Neighbor so I'm just going to reject the packet. If you were to go and debug the ISP 2 packets, the BGP packets on ISP 2 then you'd actually see the Commands Connection Refused by Remote Peer. Basically saying the other Router said you're not configured for that IP address and so I'm just going to ignore you. So in this case, we will do Update Source and we'll say the Update Source for Neighbor 1.1.1.1 will be 2.2.2.2 and that's going to be the interface that it sends it's BGP Updates and Hello requests and the other things from there. Well I obviously boofed that up. Update Source, that's right, we have to specify an interface, not just an IP address. In this case we'll do Update Source Loopback 1. And let's make sure that, that is the right IP address. Do Show IP Interface Brief, yes so Loopback 1 is the 2.2.2.2 address. So now the Access 2 Peer is configured. Let's go over to ISP 2 and set it up. We're going to use pretty much the same commands only obviously we're going to reference the other ones IP address, the Access 2 IP address. So Config T, we'll do Router BGP 50. We'll do Neighbor 2.2.2.2, Remote AS will be 12. Neighbor 2.2.2.2 EBGP Multihop 5 and then we'll say the Update Source will be Loopback 1. While we're in here we will also configure the networks the Loopback Networks that are on this Router. 150.10.1 150.11.1 and 150.12.1. So we'll do Network 150.10.1.0 255.255.255.0, I always forget that Mask Command. Mask 255.255.255.0. You see our Neighbor finally came up, it took it a little bit. So we'll advertise this network, the 11. network and the 12. network into NGP using the Network Commands. And we'll pick up with our routing exercise 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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