Shadowing Practice: BGP Overview - Learn English Speaking with Video

Hi, welcome to this latest episode of Lightboard Lessons.

Today we're going to 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 going to 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 ASs out there, Autonomouss.

I don't have the numbers memorized so we'll just say one,

two, three and let's put a fourth one out here.

And so BGP is the protocol, It's the routing protocol and it is actually a path vector protocol.

Okay.

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 AS4 is a fairly small, you know autonomous system maybe they just have two routers

but maybe in AS2 there's 150 routers in here and AS1 there's 300 and you know maybe AS3 there's 75.

So you really don't know from BGP's perspective how many routers are within an autonomous system because it's looking at the path.

So if I have a router out here on the edge

and say you know I'm a I'm a stub customer of AS4 or an AS2.

And AS4 has peering agreements with AS4 and AS2 and AS2 and AS1 have agreements and so on.

And so say there's another one out here.

We'll call this AS5.

Okay.

And so they're all connected.

So say you have a website out here that you want to go to and so on your gateway router,

in this case, this would be like maybe your mom

and pop ISP or even say this is like a charter or a cable and wireless or whatever,

you know, a smaller cable and wireless actually bought by another company.

The ISPs are constantly changing.

Say that you have your internet service, right?

So you've got your router at home and you subscribe up here.

So you're connected into their routing infrastructure and they connect out into this big cloud that we call the internet.

And so if this address exists on, let's say it's on the block and and I'm just throwing a number out.

I 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, and it's an explicit relationship.

They have to define peers between their edge routers and other autonomous system routers.

And so they'll define the neighbor relationship at all these points,

and then they will decide with policy and with configuration,

what routes to advertise out to the world and what routes to accept in from the world.

And so the routing table can be quite small within an autonomous system, but overall throughout the world, it can grow quite large.

And I forget what the size is now, but I think it's somewhere around four or 500,000.

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 wanna be round robin, load bouncing 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 as a slash 24,

that might be aggregated into say something like 125.4.0.0 and you know,

that's like a slash 20 or something like that.

And so it will summarize all the routes that include this particular one as it advertises out.

But as we look at the path to this route, Because of all my different connections,

I get that from AS4 via AS2,

then splits out AS1 and AS3.

And then from AS1 could go to AS5 and then back down to AS3.

and so you could also just go straight to AS2

and there isn't a relationship over to AS3 from AS2

so you have to hop back up to AS1 and then down to AS3

and you might have routes also for this coming from AS5 so anyway,

the path length is one of the attributes which BGP looks at to select the best path So,

if I pick one, say that my path is AS2, AS1, AS3, this one.

And then the other one, let's say, was AS4 to AS2 to AS1 to AS3.

And we'll just kill that route and kill this one.

So, my path length is 4 here and my path length is 3.

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 take.

Now 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 optimize routes by injecting or changing policies,

rather, that will influence the direction of traffic to meet customer SLAs or provide additional services.

So there is a very complex 10 to 12,

depending on how many sub-attributes that you go 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, this is how the internet works.

And these, before we leave, these relationships between ISPs, 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, you know, RFC 1918 type routes,

there's nothing preventing you from a routing perspective from sending them to me.

but obviously you're not supposed to share private address space.

And then you also have the additional concerns of potentially taking your IGP and re-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 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 a third-party module that we have in the ZebOS that you can license.

And there's a lot of neat things that you can do on the Big IP with BGP.

So thank you for joining us.

We'll see you out there in the community and make sure you subscribe.

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