自治系统商业关系--AS Relationship2
上面就体现了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