â– Intermediate System-to-Intermediate System (IS-IS)
â– Open Shortest Path First (OSPF)
â– 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.
■Keepalive messages—BGP speakers periodically exchange keepalive messages
to determine whether the underlying TCP connection is still up.
■Notification messages—If a BGP speaker wishes to terminate a BGP session
(either because it has been configured to do so or because it has detected some
error condition), it will send a notification message to its peer specifying the
reason for terminating the BGP session.
Overview â– 7
Chapter 1: Configuring BGP Routing
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