本帖最后由 tingtingyuli 于 2016-10-22 13:31 编辑
英文原文：
Why Should You Care About 4-octet AS：
At the time of BGP development and standardization, it was assumed that availability of a 16 bit binary number to identify the Autonomous System (AS) within BGP would have been more than sufficient. The 16 bit AS number, also known as the 2-octet AS number, provides a pool of 65536 unique Autonomous System numbers (ASN). From this pool, 1023 numbers have been reserved for local or private usage, and 3 are reserved for special usage. The remaining pool of AS's (64510) are available to support the public Internet infrastructure. The IANA manages the available BGP ASN pool, with the assignments being carried out by the Regional Registries.
An important aspect is the consideration regarding backward compatibility and interoperability between the `old' 2-octet AS numbers and the `new' 4-octet AS numbers. The IETF and the standardization of the revised 4-octet AS number is developed in such a way that there is a high degree of backward compatibility between the 4-octet and 2-octet AS number. This backward compatibility is achieved by creating a set of new BGP capabilities, attributes and communities.
Cisco IOS Software releases supporting RFC4893 (BGP Support for Four-octet AS Number Space) introduces the following new BGP aspects。
The difference between a full 4-octet AS system and a 2-octet AS system is negligible because the BGP operation does not change at all. The only difference is that in one case the AS is 2 Bytes, it will be in the other case, 4 Bytes. RFC4893 has succeeded for most of the deployments.
中文翻译：
你为什么要关心大约4个八位字节的AS：
在BGP开发和标准化时，我们假定16位二进制数的可用性以识别BGP内的自治系统（AS）将是足够使用的。 16位AS编号，也称为2字节AS编号，提供65536个唯一自治系统编号（ASN）池。从这个池中，1023个号码被保留用于本地或私人使用，并且3个被保留用于特殊用途。剩余的AS池（64510）可用于支持公共互联网基础设施。 IANA管理可用的BGP ASN池，分配则由区域注册管理机构执行。
一个重要的考虑是，在“旧”2字节AS号和“新”4字节AS号之间的向后兼容性和互操作性。 IETF和修订的4字节AS号的标准化是以这样思想方式开发的，即在4字节和2字节AS号之间提供高度的向后兼容性。这种向后兼容性是通过创建一组新的BGP能力，属性和团体来实现的。支持RFC4893（BGP支持四个八位字节的AS号空间）的Cisco IOS软件版本引入了以下新的BGP特征。
完整的4字节AS系统和2字节AS系统之间的差异是可以忽略的，因为BGP操作根本没有变化。 唯一的区别是，在一种情况下，AS是2字节，在另一种情况下，它是4字节。 RFC4893已经在大部分网络中成功部署。
