Configuring Bgp - Cisco WS-C3750-48PS-S Software Configuration Manual

Configuring BGP

Configuring BGP
The Border Gateway Protocol (BGP) is an exterior gateway protocol used to set up an interdomain
routing system that guarantees the loop-free exchange of routing information between autonomous
systems. Autonomous systems are made up of routers that operate under the same administration and
that run Interior Gateway Protocols (IGPs), such as RIP or OSPF, within their boundaries and that
interconnect by using an Exterior Gateway Protocol (EGP). BGP version 4 is the standard EGP for
interdomain routing in the Internet. The protocol is defined in RFCs 1163, 1267, and 1771. You can find
detailed information about BGP in Internet Routing Architectures, published by Cisco Press, and in the
"Configuring BGP" chapter in the Cisco IOS IP and IP Routing Configuration Guide.
For details about BGP commands and keywords, refer to the Cisco IOS IP Command Reference, Volume
2 of 3: Routing Protocols, Release 12.2. For a list of BGP commands that are visible but not supported
by the switch, see
Routers that belong to the same autonomous system (AS) and that exchange BGP updates run internal
BGP (IBGP), and routers that belong to different autonomous systems and that exchange BGP updates
run external BGP (EBGP). Most configuration commands are the same for configuring EBGP and IBGP.
The difference is that the routing updates are exchanged either between autonomous systems (EBGP) or
within an AS (IBGP).
Figure 34-4 EBGP , IBGP , and Multiple Autonomous Systems
AS 100
Router B
175.220.212.1
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
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:
•
Catalyst 3750 Switch Software Configuration Guide
34-40
Appendix C, "Unsupported Commands in Cisco IOS Release 12.2(20)SE."
Figure 34-4
Router A
129.213.1.2
EBGP
129.213.1.1
Figure 34-4, Routers A and B are BGP peers, as are Routers B and C and Routers C and
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.
shows a network that is running both EBGP and IBGP.
Router D
192.208.10.2
IBGP
175.220.1.2
Chapter 34 Configuring IP Unicast Routing
AS 300
192.208.10.1
EBGP
Router C
AS 200
78-16180-02