TCP/IP Routing Protocols (Gateway Protocols)
Routing is not just one of the most
important activities that takes place at the network layer: it is the
function that really defines layer
three of the OSI Reference Model. Routing
is what enables small local networks to be linked together to form potentially
huge internetworks that can span cities, countries or even the entire
globe. It is the job done by special devices called routers,
which forward datagrams from network to network, allowing any device
to send to any other even if the source has no idea where the destination
Routing is a complicated subject.
The short summary of the process is that routers decide how to forward
a datagram based on its destination address, which is compared to information
the router keeps in special routing
tables. These tables contain entries for
each of the networks the router knows about, telling the router which
adjacent router the datagram should be sent to in order for it to reach
its eventual destination.
As you can imagine, routing tables
are critically important to the routing process. It is possible for
these tables to be manually maintained by network administrators, but
this is tedious, time-consuming and doesn't allow routers to deal with
changes or problems in the internetwork. Instead, most modern routers
are designed with functionality that lets them share route information
with other routers, so they can keep their routing tables up to date
automatically. This information exchange is accomplished through the
use of routing protocols.
In this section I provide a description
of the most common routing (or gateway) protocols used in TCP/IP. I
begin with an overview of various concepts that are important to know
in order to understand how routing protocols work. I then describe the
TCP/IP routing protocols themselves in two subsections. The first covers
interior routing protocols, which are used between routers in an autonomous
system, and the second looks at exterior routing protocols, used between
If you don't understand what an autonomous
system is or the difference between an interior and exterior protocol,
then you know why I included an overview topic on concepts first. In
fact, it is really much easier to understand routing protocols once
you have a good background on the entire process of routing.
You may notice that in the title
of this section I refer to both routing protocols and gateway
protocols. These terms are interchangeable, and in fact, the word
gateway appears in the name of several of the protocols. This
is an artifact of the historical use of the term gateway in early
TCP/IP standards to refer to the devices we now call routers. Today,
the term gateway normally refers not to a router, but to a different
type of network interconnection device, so this can be particularly
confusing. The term routing protocol is now preferred, and
is the one I use.
Note: Some of the protocols in this section are generic enough that they could be applied to support the routing of any network layer protocol. They are most often associated with IP, however, as TCP/IP is by far the most popular internetworking protocol suite, and that is my assumption in describing them. Also, this section focuses primarily on the routing protocols used in Internet Protocol version 4. There is limited discussion of IPv6 versions of the protocols at this time.
Note: Strictly speaking, an argument could be made that some routing protocols don't belong in layer three. For example, many of them send messages using TCP or UDP at layer four. Despite this, routing is inherently a layer three activity and for this reason, it is traditional to consider routing protocols part of layer three.
Note: Like all topics related to routing, routing protocols are generally quite complex. I cover the major ones here in more detail than most general networking references, but even so, you should recognize that I am only scratching the surface, especially of the more complicated ones like OSPF. You can check out the referenced Internet standards (RFCs) for more details if you desire. I should also point out that there are still more routing protocols in use on IP networks that I do not cover here, such as IS-IS (which is actually an OSI protocol and not formally part of TCP/IP).
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