Routers use routing protocols to
exchange routing information with each
other. There are two types of routing
protocols, namely interior gateway
protocol, known as IGP, which is used to
refer to the process running on routers
inside of an ISP's network; and exterior
gateway protocol, known as EGP, which is
used to refer to the process running
between routers bordering directly
connected ISP networks. What is an IGP? Interior gateway protocol is used
within an autonomous system for carrying
information about internal
infrastructure prefixes. Two widely used IGPs are OSPF and IS-IS. Why do we need
an IGP? We need IGPs for ISP backbone
scaling. This is done by helping in
providing hierarchy in ISP network. IGPs also limit the scope of network
failures. IGPs are only used for ISPs
infrastructure addresses not customers
or anything else. The design goal is to
minimize the number of prefixes in IGP
to aid scalability and rapid convergence.
What is an EGP? EGP known as the
exterior gateway protocol, is used to
convey routing information between
autonomous systems. It's decoupled from IGPs. The current EGP use is called BGP.
Why do we need an EGP? Scaling for light
networks. This is done by providing
hierarchy and it also limits the scope
of network failures. EGP also defines
administrative boundaries. It is used for
applying policies on a network. For
example, it can control reachability of
prefixes, it can merge separate
organizations, and it can also connect multiple IGPs in
the network. So we're going to compare
interior versus exterior routing
protocols. With IGPs we can do automatic
neighbor discovery, whereas with EGPs
you need to specifically configure your
pairs. For IGPs you generally trust your
IGP routers whereas in EGP, you need to
connect with outside network so trust is
not generally enabled. For IGPs prefixes
go to all IGP routers, whereas in EGPs
you have to set your administrative
boundaries. IGPs usually bind routers in
one AS together, whereas in EGPs we're
binding AS's together. For IGPs, the IGP
is usually responsible for carrying ISP
infrastructure addresses only, whereas
for EGP, it carries customer prefixes as
well as it internet prefixes. IGPs
finally, for ISPs aim to keep the IGP
small for efficiency and scalability,
whereas for EGPs are independent of ISP
network topology. If you take a look at
the diagram you see that an ISP will
typically use IGPs like OSPF or IS-IS to
learn infrastructure prefixes within its
network. This ISP would use BGP to
communicate with other ISPs and IXPs
it's connected to. It would either use
static routes or BGP to communicate with
its customers. These prefixes it learns
from its customers, IXP, or other ISPs, are
typically propagated within its own
network using BGP. We will look at how
this is done in the next series.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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