Root Bridge 路径成本.PPT

冗余（Redundancy） 冗余拓扑（Redundant Topologies） 冗余拓扑（Redundant Topologies） 1.冗余拓扑的目标是为了避免网络出现单点故障 A goal of redundant topologies is to eliminate network outages caused by a single point of failure. 2.所有的网络需要冗余来提高可靠性 All networks need redundancy for enhanced reliability. 简单的冗余交换拓扑（Simple Redundant Switched Topology） 简单的冗余交换拓扑（Simple Redundant Switched Topology） 1.冗余拓扑消除了单点故障 Redundant topologies eliminate single points of failure. 2. 交换机对不知道地址的帧进行泛洪 Switches will flood frames for unknown destinations. 3.交换机对广播和组播也进行泛洪。 Broadcasts and multicasts are also flooded. 4.冗余交换拓扑或许会带来广播风暴、多帧拷贝以及MAC地址表不稳定的问题 A redundant switched topology may cause broadcast storms, multiple frame copies, and MAC address table instability problems. 广播风暴（Broadcast Storm） 多帧传输（Multiple Frame Transmissions） MAC地址表不稳定（MAC Database Instability） 创建逻辑无环路拓扑（Creating a logical loop free topology） 1.冗余增加了可靠性，但是同时将物理环路带进网络。Reliability is increased by redundancy. redundancy connections introduce physical loops into the network. 2. 解决办法就是创建逻辑无环路拓扑，同时保留物理环存在 The solution is to allow physical loops, but create a loop free logical topology. 3.无环路拓扑称为树，并且是可扩展的树。 The loop free logical topology created is called a tree. It is a spanning tree because all devices in the network are reachable or spanned. 4.创建无环路拓扑的算法称为生成树算法。 The algorithm used to create this loop free logical topology is the spanning-tree algorithm. STP术语（STP Terms） 1. 桥ID (Bridge ID) 2. 开销（Cost） 3. 桥协议数据单元（BPDU） 桥ID (Bridge ID) 开销（Cost） 最短路径是cost累加，而cost是基于链路的速率的。 Shortest path is based on cumulative link costs. Link costs are based on the speed of the link. 桥协议数据单元（BPDU） 1.交换机发送的创建逻辑无环路的数据包称为BPDU The message that a switch sends, allowing the formation of a loop free logical topology, is called a Bridge Protocol Data Unit (BPDU). 2.BPDU在阻塞的接口上也可以接收，这确保如果链路或设备出现问题，新的生成树会被计算 BPDUs continue to be received on blocked ports. This ensures that if an active path or device fails, a new spanning tree can be calculated. 3.默认，BPDU 2秒发送一次 By de