3.2.2802.1d生成树的增强特性和rstp

1、标度网络工程师系列课程Cisco Certified Network AssoteRouting and Switching(思科认证网络工程师)802.1D生成树的增强特性标度教育WWW:Q Q: 189605660;189605661BD 2008Spanning Tree Port SesForwardingLearning (forward delay = 15 sec)Link comes upListening (forward delay = 15 sec)Blocking (moves to listening after it decides if it is a root

2、port or a designated port )Blocking (loss of BPDU detected) (max age = 20 sec)生成树协议端口从阻塞状态进入转发状态必须经历两倍的Forward Delay时间，所以网络拓扑结构改变之后需要至少两倍的Forward Delay时间，才能恢复连通性如果网络中的结构变化频繁，网络会频繁的失去连通性，这样用户就会受Port fastserverPort Fast-enabled portPort Fast-enabled portsworkstationsworkstationsPortfast特性使得以太网接口一旦有设备接

3、入，就立即进入转发状态，减少收敛时间基本网络拓扑汇聚4506-A汇聚4506-B接入3560UplinkFastenabling UplinkFast with the spanning-tree uplinkfast global configuration commandswAswA (root)(root)swBswBL1L1L2L3L2L3Link failureUplinkFast transitions port directly to forwarding stateBlocked portswCswC如果交换机C在当前活动链路L2上检测到一个故障，UplinkFast将会把Sw

4、itchC的blocked port 直接转换为Forwarding状态。大概需要1-5秒。UplinkFastBackbone switchesDistribution switchesActive link Blocked linkAccess switches配置uplinkfast，当Active link出现故障时，Blocked link立即启用，直接进入Listening状态15s，再进入Learning状态15s，然后直接进入Forwarding状态。Root bridge快速生成树协议(RSTP)快速生成树协议是从生成树协议发展而来，实现的基本快速生成树具备生成树的所有功能；一致；快速生成树改进目的就是当网络拓扑结构发生变化时，尽可能快的恢复网络的连通性。RSTP生成树配置Switch(config)#Spanning-tree mode ?mstpvst raMultiple spanning t