Interior Gateway Protocols
Not all the routers within an AS have to be BGP peers. For example, in some large
enterprise networks, ASs generally have many more non-BGP routers. These routers
communicate using an interior gateway protocol (IGP) such as the following:
•
Intermediate System-to-Intermediate System (IS-IS)
•
Open Shortest Path First (OSPF)
•
Routing Information Protocol (RIP)
Figure 3 on page 7 shows that the routers in AS 53 all communicate with each other
using an IGP. Routing information internal to AS 53 is redistributed from the IGP into BGP
at router Chicago. Router Chicago redistributes into the IGP the routing information it
receives from its external BGP peer, router Atlanta. Router Atlanta has an internal BGP
link within its AS, and an external BGP link to router Topeka.
Figure 3: Interior Gateway Protocols
BGP Messages
BGP speakers exchange routing information with each other by exchanging BGP messages
over a BGP session. BGP uses the following five message types:
•
Open BGP messages—When two BGP speakers establish a BGP session with each
other, the first message they exchange after the underlying TCP session has been
established is an open message. This message contains various bits of information
that enable the two BGP peers to determine whether they want to establish a BGP
session with each other—for example, the AS number of the BGP speaker—and to
negotiate certain parameters for the BGP session—for example, how often to send a
keepalive message.
•
Update messages—The update message is the most important message in the BGP
protocol. A BGP speaker sends update messages to announce routes to prefixes that
it can reach and to withdraw routes to prefixes that it can no longer reach.
7Copyright © 2010, Juniper Networks, Inc.
Chapter 1: Configuring BGP Routing
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