MicroNugget: BGP Configuration Explained | CBT Nuggets

how exactly does BGP the border gateway protocol choose its routes in this micronet we're gonna do a brief overview of how the internet works with BGP and then specifically take a look with an example and how BGP can choose the best routes when it's presented with more than one path let's jump in so to appreciate how BGP makes forwarding decisions and chooses routes let's make sure we're comfortable what BGP the border gateway protocol does for a living in our network that we have here I have five clouds and these five clouds represent the entire internet now I know you're thinking teeth beginning it's a little bigger than this and I know that's true but for our example we'll say it's just these five these five clouds represent five different companies who are providing internet-based services and they are given something called an autonomous system number this company has been given a time system twelve for them and a time assistant number 34 for this company fifty six nine ten and seventy eight so how do we get the ball rolling with IP addresses on the internet well the first thing that happens is we have a need we have a need for addresses so there's the ia na the internet assigned numbers authority that's responsible for managing all of the ipv4 and ipv6 address space next to hand those out the IAA has hired or work with another division called the regional internet registries and there's multiple of those several of them if a company wants to go in business or they want more IP addresses they make a request their local regional internet registry they get their IP addresses so let's say for example that this autonomous system is nine ten what they want to do is they want to request a new block of addresses so they go to their regional internet registry they say can we please have a new block of addresses now maybe this was like a dozen years ago when there are plenty of ipv4 addresses available let's say they make a request and they're given this block of addresses twenty-three dot 6400 let's just dream big let's say they're given a slash 12 that means the first 12 bits are the network and everything after that are available addresses that they can then subnet as much as they want to and rent out to customers so how does this all fit in with BGP well if this autonomous system number has been given this address space to advertise and to share with the world the job of BGP is to do exactly that so once we have those networks installed inside of our autonomous system our next job is to advertise those routes to neighbors so we're going to advertise the routes to ASN 56 and we're going to advertise it up to a SN 78 and these autonomous systems will also advertise and propagate those routes all the way across the internet so what's the problem well here's the here's the challenge let's say that autonomous system 12 learns about the network 23 that's 64 so it's learned it but it's learning it from two sources from here and from here which is the best way to go should it go path a across here or should it go path B across here and this is the point of this micro nugget is to give you the decision point of how does BGP choose the route to get to the 2364 network does it go path a or b and the way it works is this when a s and 9/10 advertises the route it puts on the path 910 there's a little attribute a little value called what path has this route been advertised on when a as 56 advertises it over it tax on its autonomous system err 56 and so when this route shows up on path B it's going to show up that it went through a s 910 it went through a s 56 and here's the route on path a it's going to appear like this it's gonna have nine ten as part of the path it's gonna have 78 as part of the path and it's going to have 34 as part of the path so for path a it's gonna look like it went through nine ten and seventy eight and thirty four you with me those three autonomous systems up top and these two autonomous systems for the path till all the router does inside of BGP I shouldn't say all but the primary decision making for which path to use is by looking at the length of the path because this path up here is 3 and this path down here is 2 it's going to choose the path with the lowest number of autonomous systems in the AAS path list that's it so let's take a look at this in action let's say a ton of system 9/10 has been given that network block at 23 64 / 12 and our 10 has been chosen to inject or introduce that Network statement into BGP we could go over to our 10 to simulate this go into configuration mode for BGP for a ton of the system 910 and simply add the network statement and now he has the ability to advertise that Network now 910 to say yes is advertising it northbound to say it 78 and also to 5678 advertising it 234 and as they advertise that route they're going to append to the autonomous system path there a s number there are a ton of the system number why is that important let's go take a look at r1 and I'll show you we do a show IP BGP right here it's going to show us that we have one path for that route and now we have so bgp takes a few moments to converge I'll be quite honest I was expecting both of them to be there the first time so this is the network that we're after the 2364 zero 12 and we have two entries for it if we take a look at the far right this part right is saying that the a at the first day s it came through is nine ten because we sourced it and then it went through 56 which is right here the other route for that same the other path that same network was sourced from 910 went through autonomous systems 78 right up here and then went through a ton of the system 34 right here and now the question comes into play how does BGP choose its routes and the answer is it's going to choose the route with the lowest path that check this out right here if we take a look at this route right here or this path this one we had to go through to a s's this one had to go through three and that's why my friends this little guy right here the little greater than symbol means that is the best route this router is choosing the route that went through the two autonomous systems as opposed to the one that went through the three all because of the a s path the lowest is the best and that my friends is how BGP chooses its best path for a route I hope this has been informative for you and I'd like to thank you for viewing

Channel: CBT Nuggets

Views: 144,013

Rating: 4.9367361 out of 5

Keywords: cbt nuggets, routing protocols, cisco training, computer based training, bgp configuration, bgp explained, how bgp works, how to configure bgp, bgp example, bgp in networking, bgp fundamentals, bgp introduction, what is bgp protocol and how it works, bgp route types, cbt nuggets ccna, jeremy cioara, cbt nuggets review, cbt nuggets security+, cbt nuggets network+, cbt nuggets comptia a+

Id: XRetkD4UUL4

Length: 7min 18sec (438 seconds)

Published: Mon Jul 09 2012