PLANET XGS3-24042 - Manual (Page 335)

40-1
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