Cisco Catalyst 9500 series Configuration Manual page 126

BGP Network Topology
is that the routing updates are exchanged either between autonomous systems (EBGP) or within an AS (IBGP).
The figure given below shows a network that is running both EBGP and IBGP.
Figure 6: EBGP, IBGP, and Multiple Autonomous Systems
Before exchanging information with an external AS, BGP ensures that networks within the AS can be reached
by defining internal BGP peering among routers within the AS and by redistributing BGP routing information
to IGPs that run within the AS, such as IGRP and OSPF.
Routers that run a BGP routing process are often referred to as BGP speakers. BGP uses the Transmission
Control Protocol (TCP) as its transport protocol (specifically port 179). Two BGP speakers that have a TCP
connection to each other for exchanging routing information are known as peers or neighbors. In the above
figure, Routers A and B are BGP peers, as are Routers B and C and Routers C and D. The routing information
is a series of AS numbers that describe the full path to the destination network. BGP uses this information to
construct a loop-free map of autonomous systems.
The network has these characteristics:
• Routers A and B are running EBGP, and Routers B and C are running IBGP. Note that the EBGP peers
are directly connected and that the IBGP peers are not. As long as there is an IGP running that allows
the two neighbors to reach one another, IBGP peers do not have to be directly connected.
• All BGP speakers within an AS must establish a peer relationship with each other. That is, the BGP
speakers within an AS must be fully meshed logically. BGP4 provides two techniques that reduce the
requirement for a logical full mesh: confederations and route reflectors.
• AS 200 is a transit AS for AS 100 and AS 300—that is, AS 200 is used to transfer packets between AS
100 and AS 300.
BGP peers initially exchange their full BGP routing tables and then send only incremental updates. BGP peers
also exchange keepalive messages (to ensure that the connection is up) and notification messages (in response
to errors or special conditions).
In BGP, each route consists of a network number, a list of autonomous systems that information has passed
through (the autonomous system path), and a list of other path attributes. The primary function of a BGP
system is to exchange network reachability information, including information about the list of AS paths,
with other BGP systems. This information can be used to determine AS connectivity, to prune routing loops,
and to enforce AS-level policy decisions.
Routing Configuration Guide, Cisco IOS XE Everest 16.6.x (Catalyst 9500 Switches)
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Configuring IP Unicast Routing