Accton Technology ES4626 - Manual (Page 511)

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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 ES4624-SFP/ES4626-SFP 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.