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So let's begin our journey into BGP by covering some of its basic attributes

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and some of the

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tools that we use when troubleshooting BGP.

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Now keep in mind, the number one tool in our toolbox when troubleshooting

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anything is to

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have knowledge of what it is we're trying to troubleshoot.

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The more we know about it, the more we know how it normally behaves, making it

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easier

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to pick out why it's not behaving normally and what might be causing that.
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to pick out why it's not behaving normally and what might be causing that.

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So that said, this is hopefully not your first introduction to BGP.

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We are hoping that you have some base knowledge of BGP before we jump in.

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If not, I would encourage you to review other material before we jump in here.

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We're hoping that this is going to serve as a reminder of some information you

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already

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knew.

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So that said, let's jump in and get started looking at some of the basic

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characteristics

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and attributes of BGP.
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and attributes of BGP.

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So the first thing we want to be aware of with BGP is that this is in fact a

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path vector

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routing protocol.

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And essentially what that means is that we're going to be routing the traffic

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between different

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autonomous systems or AS's and we're going to be using the shortest path in

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relation

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to our AS numbers as we pass the routes to the environment.

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Now BGP of course uses a rather complex route selection criteria, which we're
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Now BGP of course uses a rather complex route selection criteria, which we're

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going to be

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taking a look at later.

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And part of that is the shortest AS path.

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But at this point, we're just trying to point out the fact that we are routing

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between

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AS's, we are not routing between next hops or routers or devices.

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However, you want to say that we're routing between autonomous systems.

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So to a certain extent, BGP is more like the 30,000 foot view when it comes to
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So to a certain extent, BGP is more like the 30,000 foot view when it comes to

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routing.

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This is why it's considered an EGP or an external gateway protocol.

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It's not meant for inside of a network.

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Now we can of course certainly use it for that, particularly in regards to some

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of its

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extensions that have come out over the years making this multiproticall.

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What we mean by that is of course it supports IPv4.
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What we mean by that is of course it supports IPv4.

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Wouldn't be much of a routing protocol for the internet if it wasn't.

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And just in case you weren't aware, yes, BGP is the routing protocol used on

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the internet

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for routing between service providers and so on.

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It of course also supports IPv6.

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So once IPv6 becomes more prevalent and that is happening more and more as time

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goes by,

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BGP is ready to go, fully supports IPv6.
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BGP is ready to go, fully supports IPv6.

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But it's also been extended to support other things such as MPLS or what we

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call VPN v4

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or VPN v6 routes.

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So essentially what that means is for MPLS this is acting as the control plane.

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And we have the same thing for VXLAN or virtual extensible LAN.

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In this case it's being used to exchange MAC addresses and such, but
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In this case it's being used to exchange MAC addresses and such, but

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nonetheless it

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is still acting as a control plane for VXLAN.

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So BGP has a lot of different uses, of course the most popular and the most

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famous of all

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is internet routing.

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But it's also used as the control plane in MPLS and VXLAN.

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And a lot of these are for the reasons that we're going to be talking about

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next.

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Let's talk about some of the benefits of using BGP.

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First for most and probably its biggest and most important feature, it is
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First for most and probably its biggest and most important feature, it is

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scalable.

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BGP is the most scalable routing protocol that we have, being able to handle,

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particularly

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when we get to things like MPLS cores, carrying customer routes and so on, it

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can scale to

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literally millions of routes.

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So very, very scalable.

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Along with scalable is always good to have stability.

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BGP is also very stable.

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Now what exactly does that mean?
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Now what exactly does that mean?

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Well, BGP particularly inside of an environment, so our IBGP, neighborships are

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often built

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between loopback interfaces.

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That way, if there are multiple physical paths between devices and one of those

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paths were

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to go down, particularly the one we're using, the underlying IGP.

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So for example, maybe OSPF or ISIS.

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That routing protocol will converge so fast that BGP never even realizes that
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That routing protocol will converge so fast that BGP never even realizes that

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its neighbor

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went down.

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Therefore, we don't have to rebuild a neighborship.

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We don't have to re-synchronize our BGP tables.

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Everything just stays stable.

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And honestly, another huge feature is the ability to do route filtering and

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route control.

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But let's start with filtering.

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We can of course do both inbound and outbound filtering.

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And we can do this with things like ASPath lists, ACLs, prefix lists, route
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And we can do this with things like ASPath lists, ACLs, prefix lists, route

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maps, and

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even just setting something like a prefix limit, which simply says you may not

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accept

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more than 10,000 routes from this neighbor.

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That way, if the neighbor accidentally gets misconfigured and tries to send you

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100,000

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routes, we'll shut down that neighborship.

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Now as far as the filtering goes here, we're going to get into more detail on

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this filtering

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in the next skill.
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in the next skill.

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But for now, let's just be aware that BGP does have some very powerful route

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filtering

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capabilities.

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And right along with that is our traffic engineering or path selection.

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So what we mean by this is simply manipulating BGP so the traffic goes the way

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we want it

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to.

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Now, the way we're going to do this is usually with route maps and those route

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maps are going

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to be used to manipulate the BGP attributes.
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to be used to manipulate the BGP attributes.

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The most direct attribute to change to pick a particular path would be to

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change the next

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hop.

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If you change the next hop of a route, you are very, very clearly choosing

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which way that

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traffic's going to go.

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However, we can use some of the other BGP attributes such as local preference,

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the AS

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path length, the Med or the metric as Cisco calls it, Cisco's proprietary
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path length, the Med or the metric as Cisco calls it, Cisco's proprietary

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weight attribute,

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any of these can be manipulated to affect BGP's best path choice as far as

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which route

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to choose and which one to put into the routing table.

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And of course, we'll talk about the order these are processed in and the best

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path selection

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criteria coming up later in this skill.

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Also we put community strings on here.

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Just be aware that community strings are really just an indirect way to affect

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these other
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these other

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attributes.

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In other words, we can have our route map match on community strings and based

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on the

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community strings, then change one of these other attributes, therefore

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affecting the

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path selection.

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So it's sort of an indirect method, but it's a good way of implementing a

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policy so that

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all routes with certain community strings get treated the same way.

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So to wrap up our introduction here, let's take a look at some of the tools

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that are
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that are

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very helpful when troubleshooting BGP.

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Now the first one, of course, isn't directly related to BGP itself.

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Just the good old show IP interface brief to make sure the appropriate

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interfaces are

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up.

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Basically just troubleshooting 101 on the router.

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Then we have show TCP brief all.

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This is a way to make sure that your device is listening to the appropriate TCP

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port.

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Remember, we would want to be listening to TCP port 179.
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Remember, we would want to be listening to TCP port 179.

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Show IP BGP summary is a great way to get a basic overview of our neighborhoods

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, whether

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the neighbors are up what their current status is, and how many prefixes have

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been exchanged.

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Also, of course, a list of what devices we expect to be our neighbors.

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For more detail, we can do a show IP BGP neighbor.

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So if we don't get enough information from summary, of course, neighbor is

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going to give

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us a lot of details.

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And we can, if we want to after this, specify the address of the neighbor and
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And we can, if we want to after this, specify the address of the neighbor and

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get even more

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information.

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We have a show IP route BGP, of course, to make sure BGP routes are showing up

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in the

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routing table.

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Show IP BGP, of course, this is how we look at the BGP table.

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This is going to give us a lot of important information.

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For one thing, we can see all those attributes we were just talking about.

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Plus, we can see if it's been selected as the best path, if the next hops reach
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Plus, we can see if it's been selected as the best path, if the next hops reach

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able,

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all that kind of information.

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Then, of course, we have some good debug's as well.

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Debug IP TCP transactions.

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We can use this to verify that we're actually attempting to open a TCP three

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way handshake

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to the appropriate neighbor and that we're trying to open it on port 179 once

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again.

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And debug IP BGP events.

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Also a very good debug to watch what's going on with BGP itself, such as

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neighborships

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forming and tables being exchanged and so on.
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forming and tables being exchanged and so on.

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Now as per usual, there's a lot more show commands and debug commands.

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This is just meant to be a quick list of some of the most popular.

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So in this video, we just jumped in and got started with an overview of some of

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the more

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important BGP attributes, where we would use it, what BGP is good for.

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Then of course, we took a look here at some popular tools that we're going to

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be using

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for troubleshooting BGP issues.
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for troubleshooting BGP issues.

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And of course, one of the primary issues would be neighborships not forming

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properly.

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And that's what we're going to take a look at in the next video.

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