Planet XGS3-24042 User Manual download pdf (Page 335)

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Chapter 40 BGP

40.1 Introduction to BGP

BGP stands for a Border Gateway Protocol.It’s a dynamic routing protocol inter-autonomous system. Its basic

function is automatically exchanging routing information without loops. By exchanging routing reachable

information with autonomous number of AS sequence attributes, BGP could create autonomous topological

map to eliminate routing loop and implement policies configured by users. Generally, the switches in an AS

may use several IGPs (Interior Gateway Protocol) in order to exchange routing information in the AS, such as

RIP and OSPF which are IGPs; and exchange information among ASes with EGP (Exterior Gateway

Protocol). For example, BGP is one kind of EGP. The AS is usually established on a single administrative

department. BGP is often used on the switches among ISPs or the departments of Multi-national Corporation.

BGP has been used since1989, its earliest three versions are RFC1105（BGP-1）、 RFC1163（BGP-2）and

RFC1267（BGP-3）.Currently, the most popular one is RFC1771（BGP-4）. The switch supports BGP-4.

1．Characteristics of BGP-4

BGP-4 is suitable for the distributed structure and supports Classless InterDomain Routing (CIDR). BGP-4 is

becoming the virtual exterior routing protocol standard used for the global Internet. The features of BGP-4 are

as follows.

 BGP is an exterior routing protocol, unlike interior routing protocol, such as OSPF and RIP, BGP

can’t discovery and calculate routes, but it can control the transmission of routes and select the best

route.

 By carrying AS routing information in the updating route, the problem of Routing Loops can be

resolved

 BGP uses TCP on port 179 as its transport protocol, this could enhance the reliability of the protocol.

 BGP-4 supports CIDR (Classless InterDomain Routing), which is an important improvement to

BGP-3. CIDR has a brand new way to look on IP address; it doesn’t distinguish class A , Class B

and class C network. For instance, an illegal class C address 192.213.0.0 255.255.0.0 can be

represented as 192.213.0.0/16 by CIDR which is a legal super network. /16 represents that the

network number is formed by 16 bits from the beginning left of the address. The introduction of CIDR

abbreviates the route aggregation. The route aggregation is the process of combining several

different routes. So notifying several routes can be changed to notify only one route which

decreases the route table.

 When updating route, BGP send only incremental route. The bandwidth occupied by BGP

transmission is reduced greatly and it is suitable for the mass routing information transmitted on the

internet

 For political and economical reasons, each AS expects to filter and control the route, BGP-4

provides abundant route policies which make BGP-4 more extendable to encourage the internet

development.

2． The Overview of BGP-4 operation

Unlike RIP and OSPF protocols, BGP protocol is connection oriented. BGP switches must establish

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