BGP概述.ppt

自主系统号－AS 　　AS是一个16位的数字，其取值范围从1－65535；64512－65535的AS号保留给私用，这类似于私有IP地址。 　　仅当组织打算使用诸如BGP等EGP来连接到互联网时，才需要使用IANA分配的自治系统号。 　　在互联网上有15000多个AS号；有20000多台路由器在运行，均与有限数量的BGP路由器建立邻接关系。 　　Chinanet骨干网申请的自治域号为4134，Chinanet作为一个独立的自治域，采用BGP路由协议与国际internet及各省接入网交换路由信息。 何时选择BGP?何时不选择BGP? BGP is most appropriate(恰当) when at least one of the following conditions exists: An AS allows packets to transit through it to reach other Autonomous Systems (AS允许数据包穿过它到达其他自治系统). An AS has multiple connections to other Autonomous Systems.(多条外部连接,多个运营商之间；拥有多条到互联网的连接时。) Routing policy and route selection for traffic entering and leaving your AS must be manipulated.(必须对进入和离开AS的数据流进行控制.) BGP is not always appropriate. Do not use BGP if you have one of the following conditions: Single connection to the Internet or other AS(仅单一连接) Lacks memory or processor power to handle constant updates on BGP routers (内存和CPU性能不佳时) Limited understanding of route filtering and BGP path selection process(没有能力实施路由过滤或BGP路径选择时.) Low bandwidth between Autonomous Systems(自治系统间带宽较低时.) BGP Characteristics (特征) BGP is a distance-vector protocol with the following enhancements: Reliable updates: BGP runs on top of TCP (port 179;可靠通信.) Incremental, triggered updates only(增量更新,触发更新) Periodic keepalive messages to verify TCP connectivity(周期性通过keepalive包发送消息.) Rich metrics (called path vectors or attributes,丰富的计量值,一一比较,BGP中称为属性;类似于IGP中的开销值.) Designed to scale to huge internetworks (e.g., the Internet;专门为大型互联网络而设计.) 支持VLSM可变长子网掩码。 BGP Databases(数据表) Neighbor table List of BGP neighbors(BGP对等体,邻居,交换路径信息,与RIP类似.) 可以直连可以不直连. 邻居关系建立在TCP连接之上. BGP forwarding table/database List of all networks learned from each neighbor Can contain multiple pathways to destination networks Database contains BGP attributes for each pathway IP routing table List of best paths to destination networks(列出了到目标网络的最佳路径) BGP表 运行BGP的路由器保存着一个独立于IP路由表的BGP表. SHOW IP BGP BGP table version is 23, local router ID is 192.168.1.49 Status codes: s suppressed, d dampe