BGP Overview

I welcome to this latest episode of life board lessons and today we're gonna talk about the border gateway protocol and so what is the border gateway protocol that is BGP and if you watched MacGyver in the 1980s he could fix anything with duct tape and chewing gum and so if DNS is the chewing gum of the internet BGP would be the duct tape it's kind of what holds everything together and so you know we're gonna do a very high level of what BGP is and how it works and then you know leave it to the viewer to go out and discover additional details but BGP really is a tool of Internet service providers and that's because you have all of these autonomous systems out here and so like an AT&T or a Savas or CenturyLink Verizon level-3 you have all these AAS is out there autonomous is I don't have the numbers memorized so we'll just say 1 2 3 and let's let's put a 4th one out here and so BGP is is the protocol it's the routing protocol and it is actually a path vector protocol ok and the way the BGP works is that you exchange routing advertisements between peers so if you have routers here at the edges of all of these autonomous systems and so you could have multiple routers you know say a s4 is is a fairly small you know autonomous system maybe they just have two routers but maybe in a s2 there's 150 routers in here and a s1 there's 300 and you know maybe a s3 there's 75 so you really don't know from BGP sper spective how many routers are within on autonomous system because it's looking at so if I have a router out here on the edge and say you know I'm a I'm a stub customer of s4 and a s2 and a s4 has peering agreements with a you know a s for a s 2 and s 2 and n is one have agreements and and so on and so say there's a another one out here we'll call this a s 5 okay and so they're all connected so say you have a website out here that you want to go to and and so on your gateway router in this case this would be like maybe your mom-and-pop ISP or or even say this is like a charter or a Cable & Wireless or whatever you know a smaller Cable and Wireless actually bought by another company that the high speeds are constantly changing but say that you have your internet service right so you've got your little your router at home and you subscribe up here and so you're connected into their routing infrastructure and they connect out into this this big cloud that we call the Internet and and so if if this address exists on let's say it's on the block and this is I'm just throwing a number out have no idea whose address this is so I apologize in advance so I have that network well the peering agreements between all of these autonomous systems is you know it's in it's a in its explicit relationship they have to define peers between their edge routers and other autonomous system routers and so they'll define a neighbor relationship at all these these points and then they will decide with policy and and with configuration what routes to advertise out to the world and what routes to accept in from the world so the routing table can be quite small within an autonomous system but overall throughout the world it can it can grow quite large and I forget what the size is now but I think it's somewhere around four or five hundred thousand it's really really large and so smaller routers to hold all of that in memory is challenging so some like in this case they may just want to be round-robin load balancing outbound they may not take full routes from either provider they may take a partial route out to the Internet to be able to conserve space but anyway back to our example this autonomous system will advertise this route out to the world now they may do that as a summary route it might not go to the world outside of this autonomous system is a slash 24 that might be aggregated into say something like 125 4.00 and you know that's a like a slash 20 or something like that and so it will summarize all the routes that include this particular one is it advertises out but as we look at the the path to this route because of all my my different connections I get that from a s4 via a s to B then splits out a s1 and s3 and then from a s1 could go to a s5 and then back down to a s3 and so you could also just go straight to a s2 and there isn't a relationship over to a s3 from a s2 so you have to hop back up to a s1 and then down to a s3 and you may have routes also for this coming from a s5 so any way that the path length is one of the attributes at which BGP looks at to select the best path so if I pick one say that my path is s2 a s1 yes three this one and then the other one let's say was a s for to a s to to a s1 to a s3 and we'll just kill that route and kill this one so my path length is four here and my path length is three so if I don't have other local weights or local preferences impacting the difference between these two then the shortest path is going to win and that's the path that I'm going to to take now the the key thing to understand about best path with BGP is that it doesn't necessarily mean it's the most performant you could have with a shorter path many more router hops than a longer path because again it's looking at autonomous system hops not router hops and so there's all kinds of metrics that some ISPs will do to look at path metrics and and optimize routes by injecting or changing policies rather that will influence the direction of traffic to meet customer SLA Zoar provide additional services so there is a very complex 10 to 12 depending on how many like sub attributes that you go in in the BGP routing the decision algorithm and so I'll leave it to you to go out and look at that but in a nutshell that this is how the internet works and these before we leave these relationships between ISPs it's it's really built on trust and so I tell you that I'm going to advertise routes you tell me you're going to advertise routes to me and if I don't put controls in there to block routes that I don't want to receive like like you know RFC 1918 type routes there's nothing preventing you from a routing perspective from sending them to me but you know obviously you're not supposed to share private address space and and then you also have the additional concerns of potentially taking your IGP andrey injecting it into your EGP in a way that might make one particular router on the internet look like the best path for everybody that's happened in the past and brought the router or brought the Internet to its knees so Trust is a big deal between all these ISPs and how they exchange routes and there's so much more that we could go into BGP but I'll leave you with that and you know BGP is supported on the big IP you can get it through with a third-party module that that we have in the Zev OS that you can that you can license and there's a lot of neat things that you can do on the big IP with with BGP so thank you for joining us we'll see out there in the community and make sure you subscribe we'll see you later you

Channel: F5 DevCentral

Views: 131,236

Keywords: f5, devcentral, bgp, lightboard, border gateway protocol explained, border gateway protocol, routing protocols, border gateway protocol (bgp), border gateway protocol facebook, facebook, instagram, facebook outage, dns, what is bgp, what is border gateway protocol

Id: _Z29ZzKeZHc

Length: 9min 41sec (581 seconds)

Published: Wed Jan 10 2018