TCP/IP Tutorial and Technical Overview - IBM Redbooks

► Routes and paths: A route associates a destination with a collection of
attributes describing the path to the destination. The destination is specified in
NRLI format. The path is reported as a collection of path attributes. This
information is advertised in UPDATE messages. For additional information
describing the UPDATE message, see 5.9.3, “Protocol description” on
page 220.
5.9.2 IBGP and EBGP communication
BGP does not replace the IGP operating within an AS. Instead, it cooperates with
the IGP to establish communication between autonomous systems. BGP within
an AS is used to advertise the local IGP routes. These routes are advertised to
BGP peers in other autonomous systems. Figure 5-22 on page 219 illustrates
the communication that occurs between BGP peers. This example shows four
autonomous systems. AS 2, AS 3, and AS 4 each have an EBGP connection to
AS 1. A full mesh of IBGP sessions exists between BGP devices within AS 1.
Network 10.0.0.0/8 is located within AS 3. Using BGP, the existence of this
network is advertised to the rest of the environment:
► R4 in AS 3 uses its EBGP connection to announce the network to AS 1.
► R1 in AS 1 uses its IBGP connections to announce the network to R2 and R3.
► R2 in AS 1 uses its EBGP session to announce the network into AS 2. R3 in
AS 1 uses its EBGP session 5 to announce the network into AS 4.
218 TCP/IP Tutorial and Technical Overview