AS relationship of BGP policy

[align=left]商业关系的导出策略所体现出来的公认的valley-free原则，用形象的话说就是：[/align]
[align=left]The above export-policy is called valley-free because if all nodes in[/align]
[align=left]a route adhere to this policy, the resulting route consists of a sequence[/align]
[align=left]of customer-provider traversals to a peak which may consist of zero or[/align]
[align=left]one peer-peer traversals, followed by a sequence of provider-customer[/align]
[align=left]traversals.[/align]
[align=left]解释得书面化点就是：[/align]
[align=left]Even though each AS will ultimately know about routes to all prefixes[/align]
[align=left]in the BGP network, it only makes certain routes visible to each[/align]
[align=left]of its neighbors. The decision about which routes are made available[/align]
[align=left]to which neighbor is usually determined by an AS’s relationship to its[/align]
[align=left]neighbor due to cost considerations and responsibilities to neighbors.[/align]
[align=left]An AS needs to make its customer’s prefixes visible to the world, so it[/align]
[align=left]typically exports customer-routes to its providers. Similarly, it needs[/align]
[align=left]to make the prefixes of all ASes visible to its customers, therefore[/align]
[align=left]makes all its available routes visible to its customers. In routing traffic[/align]
[align=left]to neighbors, traffic to a provider is usually the most expensive, so[/align]
[align=left]ASes form peer-peer relationships to avoid accessing each-other’s[/align]
[align=left]customers through providers. Therefore peers export customer-routes[/align]
[align=left]to each other. Additionally since traffic to providers and peers do[/align]
[align=left]involve costs, ASes do not typically carry provider-provider traffic or[/align]
[align=left]peer-peer traffic, and so do not export provider-routes or peer-routes[/align]
[align=left]to other providers or peers[/align]
[align=left] [/align]
对于上面的商业关系，一般使用不同的local-preference值来控制来自不同商业伙伴的路由的优先级。Local-preference值一般使用route-map基于前缀或者AS-Path或者邻居自治系统来进行设置和分配，研究表明，大多数AS是基于邻居自治系统(的商业关系)来设置local-preference