In a autonomous system 20 and 40 and 30, each of the interior IBGP routers in
those autonomous
systems are advertising their loop back.
So down here it's 32.32.32, etc. with a 32-bit mask and up here it's 22.22.22
with a 32-bit
mask and down here it's 43.43, etc. with a 32-bit mask.
And that represents the loop back on each of those respective routers.
And those are being advertised into BGP and that's why over here at R12 and R13 WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And those are being advertised into BGP and that's why over here at R12 and R13
and R11
that's why they know about those networks is because those routers are
advertising those
networks.
So in this video I'd like to share with you one way of advertising a route or
including
a route inside of BGP.
Now we're going to use router 12 as an example for this and router 12 has a
loop back interface
of 12.12.12.12 with a /32-bit mask.
Now one of the interesting things about advertising a route in a BGP router is
that if we say
we want to advertise this route but yet we don't have a route to 12.12.12.12. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
we want to advertise this route but yet we don't have a route to 12.12.12.12.
It's not going to be advertised in BGP either.
So we want to make sure that the router has access to and can reach the
networks for which
it's advertising.
So the syntax in router BGP configuration mode to advertise a network would be
router BGP
in the case of R12.
It's an autonomous system 10 that hasn't changed.
And then once we're in router configuration mode we simply type in network
space and the
network would advertise 12.12.12.12 space the keyword mask and then the mask WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
network would advertise 12.12.12.12 space the keyword mask and then the mask
and dotted decimal
that we want to use.
So if this is a 32-bit route the mask would be 255 times 4 meaning all four oct
ets would be
255.
And once we've done that and assuming R12 has that route which it does it's a
loop back
in interface address it'll then go ahead and advertise that as part of BGP.
So that'll be advertised to R13 and R11 and then R11 will advertise that to its
external
BGP neighbors it'll include information such as the AS path and also the next
hop information
and then R31 will propagate that to R41 R21 will propagate that over to R42 and WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
and then R31 will propagate that to R41 R21 will propagate that over to R42 and
then R42
and R41 will share that via their internal BGP membership connection.
And that route would also be passed down to the internal peers here here and
here.
So without further ado let's go ahead and advertise that network right here on
R12.
So here on R12 if we do a show IP BGP so I verify that is not in the BGP table
and if we go
over to another router let's take a look at our topology.
Let's go over to our how about R43 at the bottom right hand side of our top WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Let's go over to our how about R43 at the bottom right hand side of our top
ology.
We've got R43 and to a show IP BGP.
Even though it's the route for the 12-12-12 is not there but it does add the
loop backs
for R22, R32 and R43.
So let's go up to R12 and here in R12 let's go into a show run, right section B
GP.
Alright so we're going to go into configuration mode we'll go into router BGP,
autonomous
system 10 and then from here we're going to use the network statement and we WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
system 10 and then from here we're going to use the network statement and we
want to go
ahead.
In fact let's just do a quick check of the interface do show IP interface brief
please exclude
any unsigned interfaces and there's our loop back address right there and that
happens
to be a 32 bit route.
So let's go ahead and type in network and router BGP configuration mode network
and I'll
copy paste that address in and then we'll use the keyword mask space and then
the mask
which is 32 bits we'll put that in in dot a decimal and boom we are done.
So BGP is not the fastest routing protocol to converge but it's not built for WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So BGP is not the fastest routing protocol to converge but it's not built for
speed
it's built for volume to be able to handle hundreds of thousands of routes.
So now if we do a show IP BGP locally here on router 12 and those here it's in
the BGP
terminal.
So it has the next top of 0, 0, 0, 0 meaning we added this route into BGP on
this local
router and it considers that the best route.
So showing up as a valid route.
So now if we were to go to another router let's go down to R43 in the bottom
right hand corner
it should also know about that route to 12, 12, 12, 12. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
it should also know about that route to 12, 12, 12, 12.
However it's going to see the next top as R42's risk is going to learn that way
it's
also going to see it coming in from R41.
So it's going to have a route in there twice because it learned that twice from
R41 and
R42 and for the next top because I've also done the next top self command on R
41 and
R42 for its internal peer here R43.
As a result it's going to show the appropriate address R42's dress or R41's
dress as the
next top inside the BGP routing information base.
So let's head over to R43 and take a look. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So let's head over to R43 and take a look.
So there's the tab for R43 and let the up here a key it was.
Those were the three previous BGP routes.
We now have that route right here and we have that route twice coming from
those two
next tops R41 and R42 and the one that's selected as the best is this one right
here with
the next top of R41.
So one of the questions might be well why did it choose that second one and not
the first
one and let's do a quick reminder on some of the decision points the router is
going
to make as it's considering which one is the best path. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
to make as it's considering which one is the best path.
In a Cisco world the first consideration is wait so we look at the wait the
wait is zero
for both of them.
The next decision point is local preference which we look at the local
preference for
those two routes.
They are also the same of 100 each.
The next consideration would be is the route sourced by me meaning the next top
is 0,
0, 0, 0 and neither of those next tops is 0, 0, 0, 0.
But check this out the loopback interface that R43 is putting in to BGP it does
have the
next top but that applies to that route but not the 12, 12, 12 so that's all WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
next top but that applies to that route but not the 12, 12, 12 so that's all
the same.
And the next consideration is the shortest AS path and for the AS path that AS
path has
two in both of them.
So as far as which one shorter they both have two AS paths that they've gone
through so
that's equal.
And there's several other considerations making including the multi exit
discriminator
also referred to as the metric.
It was looking for the lowest one there but that met is 0.
One of the other considerations is this being learned from an external BGP
neighbor or WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
neighbor or
from an internal now from the perspective of R43 both R41 and R42 are both
internal BGP
neighbors so that's also equal.
And so as it's going down this pecking order trying to decide which one is the
best path
the tie breaker in this example in this case it boils down to the router ID or
read for
short.
So the router ID as well as the next top based on the loopback here on R41 is
41, 41, 41,
and the router ID for R42 as well as its loopback zero interface address is 42, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
and the router ID for R42 as well as its loopback zero interface address is 42,
42, 42, 42,
42.
So it's boiling down to the lowest.
However, if there was any of these other parameters that were going to tip it
off first it
would use one of those but everything up to this point was equal.
That's why it shows the lowest router ID as the decision point for which one it
was going
to consider as best.
And I would say at this point in our journey with the world of BGP you don't
have to memorize
like the 12 or 13 decision points it has to make but I would encourage you to
be very aware
of those first four in order on a Cisco router when a router is trying to WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
of those first four in order on a Cisco router when a router is trying to
decide which route
from the BGP table it should use as the best route and that's the route it's
going to
put in its routing table.
So now here on router 43 this route with the next top of 41 etc that should be
the BGP
learned route in the routing table for 12, 12, 12, 12.
And we can verify that by doing a show IP route and we just filter it for BGP
learned
routes and there's the 12, 12, 12 network with the next top of 41, 41, 41, 41.
So now that we've advertised here on our 12 that local loopback zero address
and we placed WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
and we placed
that in BGP we should be able to ping from any loopback to any other loopback
on our 12,
our 22, our 43 and our 32 those are the four routes we have in BGP.
So if we go back to our 12 now and here on our 12 do a show IP BGP that hasn't
changed
fantastic so we did a ping over to say R22 by pinging 22.22.22.22 and we source
that from
our loopback zero because for the return path they have reachability to that
now that should
work and we should be able to the same thing for the others as well. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
work and we should be able to the same thing for the others as well.
So if we ping over to 32 etc and press center that shouldn't work because the
response can't
come back because we're not sourcing it from our loopbacks we'll do a control
shift
six a couple times I'll clear off my ink we'll hit the app arrow key and this
time we'll
source it from loopback zero and because they have reachability to our loopback
zero that should
work.
Let's do a more test with a trace out to 43 43 43 43 43 43 which is the loop
back zero interface
being advertised by R43 so we'll trace to that address space and we'll source WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
being advertised by R43 so we'll trace to that address space and we'll source
that as
well from loopback zero press center and there's our path to the network.
Now if we go to R41 in our topology and that is right here that's the bottom
edge route
of there in a ton of system 40 so here in R41 we do a show IP bgp and press
center for
that 12 12 12 network it's using 23 dot 30 dot 40 dot 31 as the next hop and
that is the
IP address of R31 instead of the next top of 42 42 42 42 or if we went to R11 WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
IP address of R31 instead of the next top of 42 42 42 42 or if we went to R11
and we
went to topology R11 is the edge router up in a ton of system 10 and we did a
show IP bgp
press center here showing that it's best path to get to 43 43 is to the next
top of R21
as opposed to going through R31 in a ton of system 30 and so in the next video
I'd like
to reinforce the idea that the path is chosen based on the attributes of that
route and
to do that we're going to modify the weight for incoming routes right here on R WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
to do that we're going to modify the weight for incoming routes right here on R
11 so we
can see that once we change the weights which is the highest priority looked at
it's going
to modify which routes R11 considers best so we'll do the next video and I'll
see you there