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982 PowerConnect B-Series FCX Configuration Guide
53-1002266-01
Overview of BGP4
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Overview of BGP4
BGP4 is the standard Exterior Gateway Protocol (EGP) used on the Internet to route traffic between
Autonomous Systems (AS) and to maintain loop-free routing. An autonomous system is a collection
of networks that share the same routing and administration characteristics. For example, a
corporate intranet consisting of several networks under common administrative control might be
considered an AS. The networks in an AS can but do not need to run the same routing protocol to
be in the same AS, nor do they need to be geographically close.
Routers within an AS can use different Interior Gateway Protocols (IGPs) such as RIP and OSPF to
communicate with one another. However, for routers in different autonomous systems to
communicate, they need to use an EGP. BGP4 is the standard EGP used by Internet routers and
therefore is the EGP implemented on Layer 3 Switches.
Figure 146 on page 982 shows a simple example of two BGP4 autonomous systems. Each AS
contains three BGP4 switches. All of the BGP4 switches within an AS communicate using IBGP.
BGP4 switches communicate with other autonomous systems using EBGP. Notice that each of the
switches also is running an Interior Gateway Protocol (IGP). The switches in AS1 are running OSPF
and the switches in AS2 are running RIP. Layer 3 Switches can be configured to redistribute routes
among BGP4, RIP, and OSPF. They also can redistribute static routes.
FIGURE 146 Example BGP4 autonomous systems
Relationship between the BGP4 route table and
the IP route table
The Layer 3 Switch BGP4 route table can have multiple routes to the same destination, which are
learned from different BGP4 neighbors. A BGP4 neighbor is another switch that also is running
BGP4. BGP4 neighbors communicate using Transmission Control Protocol (TCP) port 179 for BGP
communication. When you configure the Layer 3 Switch for BGP4, one of the configuration tasks
you perform is to identify the Layer 3 Switch BGP4 neighbors.
AS 1
AS 2
EBGP
IBGP
OSPF
OSPF
IBGP
IBGP
OSPF
RIP
RIP
RIP
IBGP
IBGP
IBGP
BGP4 Switch
BGP4 Switch
BGP4 Switch
BGP4 Switch
BGP4 Switch
BGP4 Switch