AS relationship of BGP policy

  

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：

The above export-policy is called valley-free because if all nodes in

a route adhere to this policy, the resulting route consists of a sequence

of customer-provider traversals to a peak which may consist of zero or

one peer-peer traversals, followed by a sequence of provider-customer

traversals.

解释得书面化点就是：

Even though each AS will ultimately know about routes to all prefixes

in the BGP network, it only makes certain routes visible to each

of its neighbors. The decision about which routes are made available

to which neighbor is usually determined by an AS’s relationship to its

neighbor due to cost considerations and responsibilities to neighbors.

An AS needs to make its customer’s prefixes visible to the world, so it

typically exports customer-routes to its providers. Similarly, it needs

to make the prefixes of all ASes visible to its customers, therefore

makes all its available routes visible to its customers. In routing traffic

to neighbors, traffic to a provider is usually the most expensive, so

ASes form peer-peer relationships to avoid accessing each-other’s

customers through providers. Therefore peers export customer-routes

to each other. Additionally since traffic to providers and peers do

involve costs, ASes do not typically carry provider-provider traffic or

peer-peer traffic, and so do not export provider-routes or peer-routes

to other providers or peers

 

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