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Autonomous Systems

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Variations of BGP terms will be used in the next two chapters. These terms are

internal BGP (iBGP) and external BGP (eBGP), also known as an interdomain

routing protocol. These are the same BGP protocol, but iBGP runs inside an

AS while eBGP runs outside an AS and connects one AS to another AS.

So what type of routing protocol is used to find the paths and connect

these autonomous systems? An Exterior Gateway Protocol (EGP). That is
exactly what BGP is, an External Gateway Protocol used to connect and find
routes to and from autonomous systems.

BGP is defined in many Requests For Comments (RFCs), which include 1771-

1774, 1863, 1965-1966, 1997-1998, 2042, 2283, 2385, and 2439. BGPv4, the lat-

est version, and autonomous systems are defined in RFC 1771.

Now we have to remember another routing protocol that learns the net-

works and can keep loops from forming in the network. In this book, there
are three mapping protocol types to remember that help to determine paths
and eliminate data loops:

Internal routing protocols These are protocols like OSPF, IGRP,
EIGRP, and RIP, which operate at Layer 3 of the OSI Reference Model.
They are used to learn the network topology on the internal network and
IP subnets to create routes that guarantee that there are no data loops in
the Layer 3 network.

External routing protocols These protocols are used to learn the net-
work topology of multiple autonomous systems or networks and connect
them with loop-free paths.

Spanning Tree Protocol This protocol is used at Layer 2 inside a seg-
ment of an AS. It ensures that the internal network topology is learned at
Layer 2 and verifies that there is a path through the network without data
loops.

BGP uses reliable session management, using TCP port 179 for triggered

UPDATE and KEEPALIVE messages to its neighbors to propagate and
update the BGP routing table. Triggered updates are updates that are sent for
a certain reason and not on a schedule.

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