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In a autonomous system 20 and 40 and 30, each of the interior IBGP routers in

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those autonomous

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systems are advertising their loop back.

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So down here it's 32.32.32, etc. with a 32-bit mask and up here it's 22.22.22

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with a 32-bit

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mask and down here it's 43.43, etc. with a 32-bit mask.

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And that represents the loop back on each of those respective routers.

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And those are being advertised into BGP and that's why over here at R12 and R13
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And those are being advertised into BGP and that's why over here at R12 and R13

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and R11

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that's why they know about those networks is because those routers are

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advertising those

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networks.

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So in this video I'd like to share with you one way of advertising a route or

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including

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a route inside of BGP.

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Now we're going to use router 12 as an example for this and router 12 has a

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loop back interface

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of 12.12.12.12 with a /32-bit mask.

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Now one of the interesting things about advertising a route in a BGP router is

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that if we say

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we want to advertise this route but yet we don't have a route to 12.12.12.12.
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we want to advertise this route but yet we don't have a route to 12.12.12.12.

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It's not going to be advertised in BGP either.

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So we want to make sure that the router has access to and can reach the

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networks for which

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it's advertising.

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So the syntax in router BGP configuration mode to advertise a network would be

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router BGP

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in the case of R12.

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It's an autonomous system 10 that hasn't changed.

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And then once we're in router configuration mode we simply type in network

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space and the

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network would advertise 12.12.12.12 space the keyword mask and then the mask
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network would advertise 12.12.12.12 space the keyword mask and then the mask

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and dotted decimal

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that we want to use.

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So if this is a 32-bit route the mask would be 255 times 4 meaning all four oct

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ets would be

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255.

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And once we've done that and assuming R12 has that route which it does it's a

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loop back

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in interface address it'll then go ahead and advertise that as part of BGP.

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So that'll be advertised to R13 and R11 and then R11 will advertise that to its

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external

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BGP neighbors it'll include information such as the AS path and also the next

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hop information

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and then R31 will propagate that to R41 R21 will propagate that over to R42 and
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and then R31 will propagate that to R41 R21 will propagate that over to R42 and

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then R42

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and R41 will share that via their internal BGP membership connection.

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And that route would also be passed down to the internal peers here here and

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here.

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So without further ado let's go ahead and advertise that network right here on

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R12.

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So here on R12 if we do a show IP BGP so I verify that is not in the BGP table

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and if we go

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over to another router let's take a look at our topology.

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Let's go over to our how about R43 at the bottom right hand side of our top
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Let's go over to our how about R43 at the bottom right hand side of our top

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ology.

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We've got R43 and to a show IP BGP.

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Even though it's the route for the 12-12-12 is not there but it does add the

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loop backs

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for R22, R32 and R43.

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So let's go up to R12 and here in R12 let's go into a show run, right section B

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GP.

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Alright so we're going to go into configuration mode we'll go into router BGP,

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autonomous

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system 10 and then from here we're going to use the network statement and we
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system 10 and then from here we're going to use the network statement and we

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want to go

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ahead.

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In fact let's just do a quick check of the interface do show IP interface brief

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please exclude

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any unsigned interfaces and there's our loop back address right there and that

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happens

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to be a 32 bit route.

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So let's go ahead and type in network and router BGP configuration mode network

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and I'll

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copy paste that address in and then we'll use the keyword mask space and then

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the mask

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which is 32 bits we'll put that in in dot a decimal and boom we are done.

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So BGP is not the fastest routing protocol to converge but it's not built for
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So BGP is not the fastest routing protocol to converge but it's not built for

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speed

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it's built for volume to be able to handle hundreds of thousands of routes.

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So now if we do a show IP BGP locally here on router 12 and those here it's in

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the BGP

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terminal.

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So it has the next top of 0, 0, 0, 0 meaning we added this route into BGP on

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this local

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router and it considers that the best route.

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So showing up as a valid route.

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So now if we were to go to another router let's go down to R43 in the bottom

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right hand corner

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it should also know about that route to 12, 12, 12, 12.
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it should also know about that route to 12, 12, 12, 12.

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However it's going to see the next top as R42's risk is going to learn that way

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it's

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also going to see it coming in from R41.

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So it's going to have a route in there twice because it learned that twice from

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R41 and

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R42 and for the next top because I've also done the next top self command on R

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41 and

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R42 for its internal peer here R43.

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As a result it's going to show the appropriate address R42's dress or R41's

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dress as the

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next top inside the BGP routing information base.

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So let's head over to R43 and take a look.
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So let's head over to R43 and take a look.

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So there's the tab for R43 and let the up here a key it was.

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Those were the three previous BGP routes.

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We now have that route right here and we have that route twice coming from

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those two

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next tops R41 and R42 and the one that's selected as the best is this one right

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here with

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the next top of R41.

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So one of the questions might be well why did it choose that second one and not

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the first

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one and let's do a quick reminder on some of the decision points the router is

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going

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to make as it's considering which one is the best path.
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to make as it's considering which one is the best path.

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In a Cisco world the first consideration is wait so we look at the wait the

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wait is zero

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for both of them.

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The next decision point is local preference which we look at the local

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preference for

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those two routes.

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They are also the same of 100 each.

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The next consideration would be is the route sourced by me meaning the next top

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is 0,

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0, 0, 0 and neither of those next tops is 0, 0, 0, 0.

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But check this out the loopback interface that R43 is putting in to BGP it does

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have the

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next top but that applies to that route but not the 12, 12, 12 so that's all
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next top but that applies to that route but not the 12, 12, 12 so that's all

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the same.

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And the next consideration is the shortest AS path and for the AS path that AS

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path has

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two in both of them.

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So as far as which one shorter they both have two AS paths that they've gone

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through so

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that's equal.

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And there's several other considerations making including the multi exit

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discriminator

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also referred to as the metric.

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It was looking for the lowest one there but that met is 0.

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One of the other considerations is this being learned from an external BGP

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neighbor or
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neighbor or

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from an internal now from the perspective of R43 both R41 and R42 are both

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internal BGP

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neighbors so that's also equal.

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And so as it's going down this pecking order trying to decide which one is the

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best path

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the tie breaker in this example in this case it boils down to the router ID or

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read for

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short.

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So the router ID as well as the next top based on the loopback here on R41 is

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41, 41, 41,

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and the router ID for R42 as well as its loopback zero interface address is 42,
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and the router ID for R42 as well as its loopback zero interface address is 42,

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42, 42, 42,

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42.

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So it's boiling down to the lowest.

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However, if there was any of these other parameters that were going to tip it

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off first it

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would use one of those but everything up to this point was equal.

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That's why it shows the lowest router ID as the decision point for which one it

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was going

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to consider as best.

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And I would say at this point in our journey with the world of BGP you don't

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have to memorize

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like the 12 or 13 decision points it has to make but I would encourage you to

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be very aware

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of those first four in order on a Cisco router when a router is trying to
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of those first four in order on a Cisco router when a router is trying to

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decide which route

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from the BGP table it should use as the best route and that's the route it's

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going to

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put in its routing table.

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So now here on router 43 this route with the next top of 41 etc that should be

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the BGP

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learned route in the routing table for 12, 12, 12, 12.

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And we can verify that by doing a show IP route and we just filter it for BGP

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learned

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routes and there's the 12, 12, 12 network with the next top of 41, 41, 41, 41.

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So now that we've advertised here on our 12 that local loopback zero address

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and we placed
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and we placed

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that in BGP we should be able to ping from any loopback to any other loopback

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on our 12,

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our 22, our 43 and our 32 those are the four routes we have in BGP.

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So if we go back to our 12 now and here on our 12 do a show IP BGP that hasn't

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changed

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fantastic so we did a ping over to say R22 by pinging 22.22.22.22 and we source

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that from

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our loopback zero because for the return path they have reachability to that

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now that should

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work and we should be able to the same thing for the others as well.
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work and we should be able to the same thing for the others as well.

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So if we ping over to 32 etc and press center that shouldn't work because the

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response can't

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come back because we're not sourcing it from our loopbacks we'll do a control

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shift

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six a couple times I'll clear off my ink we'll hit the app arrow key and this

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time we'll

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source it from loopback zero and because they have reachability to our loopback

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zero that should

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work.

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Let's do a more test with a trace out to 43 43 43 43 43 43 which is the loop

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back zero interface

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being advertised by R43 so we'll trace to that address space and we'll source
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being advertised by R43 so we'll trace to that address space and we'll source

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that as

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well from loopback zero press center and there's our path to the network.

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Now if we go to R41 in our topology and that is right here that's the bottom

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edge route

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of there in a ton of system 40 so here in R41 we do a show IP bgp and press

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center for

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that 12 12 12 network it's using 23 dot 30 dot 40 dot 31 as the next hop and

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that is the

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IP address of R31 instead of the next top of 42 42 42 42 or if we went to R11
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IP address of R31 instead of the next top of 42 42 42 42 or if we went to R11

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and we

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went to topology R11 is the edge router up in a ton of system 10 and we did a

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show IP bgp

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press center here showing that it's best path to get to 43 43 is to the next

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top of R21

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as opposed to going through R31 in a ton of system 30 and so in the next video

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I'd like

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to reinforce the idea that the path is chosen based on the attributes of that

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route and

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to do that we're going to modify the weight for incoming routes right here on R
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to do that we're going to modify the weight for incoming routes right here on R

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11 so we

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can see that once we change the weights which is the highest priority looked at

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it's going

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to modify which routes R11 considers best so we'll do the next video and I'll

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see you there