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Unformatted text preview: tween routers in different
autonomous systems. BGP is a replacement for the older EGP that was used on the
ARPANET. BGP Version 3 is defined in RFC 1267 [Lougheed and Rekhter 1991].
RFC 1268 [Rekhter and Gross 1991] describes the use of BGP in the Internet. Much of the
following description comes from these two RFCs. Also, during 1993 BGP Version 4 was
under development (see RFC 1467 [Topolcic 1993]) to support CIDR, which we describe in
A BGP system exchanges network reachability information with other BGP systems. This file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/dynamic.htm (13 of 17) [12/09/2001 14.46.53] Chapter 10. Dynamic Routing Protocols information includes the full path of autonomous systems that traffic must transit to reach
these networks. This information is adequate to construct a graph of AS connectivity.
Routing loops can then be pruned from this graph and routing policy decisions can be
We first categorize an IP datagram in an AS as either local traffic or transit traffic. Local
traffic in an AS either originates or terminates in that AS. That is, either the source IP
address or the destination IP address identifies a host in that AS. Anything else is called
transit traffic. A major goal of BGP usage in the Internet is to reduce transit traffic. An AS
can be categorized as one of the following:
1. A stub AS has only a single connection to one other AS. A stub AS carries only local
2. A multihomed AS has connections to more than one other AS, but refuses to carry
3. A transit AS has connections to more than one other AS and is designed, under certain
policy restrictions, to carry both local and transit traffic.
The overall topology of the Internet is then viewed as an arbitrary interconnection of transit,
multihomed, and stub ASs. Stub and multihomed ASs need not use BGP - they can run EGP
to exchange reachability information with transit ASs.
BGP allows for po...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12