BGP

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1. Who needs BGP?

Exterior routing protocols are used to route between autonomous systems

A Single-Homed Autonomous System (1 link per ISP, 1 ISP)

Do not use BGP within the AS in the following situations

• There is only a single connection to the Internet or other AS.
• Internet routing policy and route selection are not of concern to the AS.
• The BGP routers have insufficient RAM or processor power to handle constant updates.
• There is limited understanding of route filtering and the BGP path selection process.
• Low bandwidth link between autonomous systems
• In Figure , router A is advertising a default network into the AS through a local IGP, such as RIP. A static route affords connectivity through router B to the AS for the ISP. The ISP is running BGP and is recognized by other BGP routers in the Internet.

2. BGP Basis Operation (RFC 1772)

The Border Gateway Protocol(BGP) is an inter-autonomous System routing protocol.

BGP forms a unique, unicast-based connection to each of its BGP-speaking peers.
BGP runs over a reliable transport protocol. BGP uses TCP port 179.

BGP does not show the details of the topologies within each AS. Because BGP sees only a tree of autonomous systems, it can be said that BGP takes a higher view of the Internet than IGP, which sees only the topology within an AS. And because this higher view is not really compatible with the view seen by IGPs, Cisco routers maintain a separate routing table to hold BGP routes.   // show ip bgp

Autonomous System（自治系统）：a set of routers under a single technical administration, using an interior gateway protocol and common metrics to route packets within the AS, and using an exterior gateway protocol to route packets to other AS.   RFC 5398

How BGP works

BGP

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原文地址：http://www.cnblogs.com/elewei/p/4640716.html