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OSI Layer 3: Routed and Routing Protocols

As you can see, the 24-byte header was divided into eight segments. Each

field contains specific information. Starting from the right and working left,
you see that the first field is for the version number. This field is one byte long
and contains the OSPF version number.

Moving on, you see that the second field is also one byte long and contains

the packet type. There are five values that can be used in this field: hello,
database description, link-state request, link-state update (LSU), and link-
state advertisement (LSA).

The next field is the packet length. This field is two bytes long and

includes the packet length in bytes.

The next two fields, Router ID and Area ID, are a little longer. Each field

is four bytes long. The router ID is the source ID for the packet. Each router
in an OSPF area has a unique ID. Each AS also has an ID that is included in
the packet, so not only the packet's source router is identified, but the AS is
as well.

The remaining fields consist of the remaining space of the 24 bytes. The

fields are Checksum, Authentication type, and Authentication, respec-
tively. The checksum field, two bytes long, is used to calculate CRC infor-
mation. The authentication type field, two bytes, contains OSPF
password information or other authentication types. Finally, the authen-
tication field contains authentication information.

BGP (Border Gateway Protocol)

The Internet consists of a number of commercial networks that connect to
each other via tier-one providers such as Sprint, WorldCom/MCI/UUNet,
and others. Each enterprise network or ISP must be identified by an auton-
omous system number. This number allows a hierarchy to be maintained
when sharing route information.

We are now familiar with several IGPs (Interior Gateway Protocols), such

as IGRP, EIGRP, and OSPF. For enterprise networks to communicate with
other autonomous systems or ISPs, the IGP information has to be injected
into BGP, which is used by all network entities that compose the Internet.

BGP (Border Gateway Protocol) is an open standard protocol that was

developed and defined in several RFCs: 1163, 1267, 1654, and 1655. Com-
plete technical details can be found in these articles. One of the requirements
of BGP was that it needed to be a loop-free protocol.

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