BGB (Broader Gateway Protocol) Video Lecture | Computer Networks - Computer Science Engineering (CSE)

what is the border gateway protocol
remember that the internet actually
consists not of a network of computers
but of a network of computer networks or
autonomous systems the border gateway
protocol is the fundamental protocol
that these systems use to figure out how
to take a packet and transmit it farther
on in the Internet so the border the
border gateway protocol is fundamental
to the idea of how routing works and let
me demonstrate how it works using this
simple example so I have four computer
networks or autonomous systems here
remember that all the internal routing
inside each of these networks has to be
done by the autonomous system itself so
to use UB as an example once a packet
that's destined for a computer arrives
at the University of Buffalo the core
internet has done its work it's finished
it says ok I've given this to the router
that it's assigned to serve at the
University of Buffalo and it's now your
job it's sort of the job of the people
who administer our autonomous system to
make sure that that packet gets to its
destination so let's imagine that G's
for autonomous systems have peering
relationships with each other like this
so a is paired with C and B that means
that a the autonomous system a has
connections to this autonomous system C
and to autonomous system B and so a can
directly send packets that are destined
for this system to see and to be same
thing with these other autonomous
systems and so to some degree that
starts to look a little bit like a
typical routing diagram where I've
replaced nodes with autonomous systems
to better reflect how the Internet
routing protocols actually work so the
border gateway protocol or BGP is what
the routers in these networks
it's how they exchange information about
how what they should do with a packet
that arrives at their network because
for example B in order to reach C if
someone inside B's network wants to
transmit a packet to someone inside
sieze Network B has to either send the
packet to this autonomous system a first
or it has to transmit the packet to a
ton of the system D and the border
gateway protocol has two functions first
of all it allows the autonomous systems
on the Internet
to figure out who they can communicate
with so it has to do with reach ability
what what hosts on the Internet can I
send packets to and it also helps that
Pro that this particular router let's
say we're starting in town of the system
B it helps it make this fundamental
choice that that router has to make over
and over and over again which is should
I send the packet to D or should I send
the packet to a so let's walk through an
example so the way the broader border
gateway protocol works is that the
routers inside these autonomous systems
periodically exchange information with
each other that records or represents
the different autonomous systems that
they have direct connections to so for
example the border gateway protocol
might operate as follows the router
inside a would send a message both to
the router and B and to the router in C
and what that message would say is it
would say I know how to talk to B and I
have a one hop link to B so I'm directly
connected to B and I also know how to
talk to C and that's at this point when
the protocol is bootstrapping itself
that's what it's sense
now once B reaches receives this message
the first thing it says is well you know
what I'm directly connected to a and so
that's not interesting to me I know how
to get packets to myself I don't need to
send a packet that's destined for my own
network to a so but it does say oh this
is interesting
I'm not directly connected to C but now
I know how to get a packet to C so if I
want to send a packet to see it
maintains a table that says I know how
to get packets to B that's myself I have
a zero hop length so internally I'm
going to route things I also now get two
packets to a I knew that before where
because I'm directly connected a and I
knew how to get packets to D I got a
directly connected to D but because I
received this message from a I found out
a new piece of information which is that
now I know that there's this network see
and I know how to get a packet to see in
order to get a packet to see what I'm
going to do is I'm going to send it to a
and because a told me it has a direct
connection to see the length of the trip
from my network to C I'm going to record
as to because it requires sending it to
a and then to see that I have it again
so this is what the routing table that's
used at B is going to look like after it
receives this message using the border
gateway protocol from a so it knows
anything destined for my own network I
handled internally anything destined for
networks a or D I can send directly to
those networks and if I need to send
something to Network C I can transmit it
through network a now all what the way
the border gateway protocol works is
that these routers are periodically
sending this information and so what B
would do is once it receives this
information it would send a message to D
with this all of this information DS
going to receive this information and
once D receives the information what DS
routing table is going to look like is
it's going to say I know how to route
things to myself I knew how to route
things to B and to see before but now
what I'm going to discover is somebody
knows how to get a packet to a and that
network is B right so if I want to send
a packet to a I transmit it to be and
that's a to hop link because B is going
to then transmit it on to a once all
these routers have exchanged information
one interesting thing to notice is that
B obviously has two routes to get
packets to see one is to send it through
a the second one is to send it through D
and choosing between those B could
choose to send packets in both
directions depending on load B could
also might have some other way of
breaking ties between these networks
that might say well I prefer to send
things to network D because my
connections a bit faster or something
like that and so I'm going to do it this
way so this is how the border gateway
protocol a very simple example how the
border gateway protocol uses these
advertisements about reach ability to
deter
to allow each host on the Internet
to build up a routing table that
essentially allows it to route to any
autonomous system that's connected to
the Internet so in order to route data
the routers that you be actually have to
know which direction to send packets
that are potentially destined for every
autonomous system that makes up the
Internet