1、實施生成樹實施生成樹冗余拓撲的設計冗余拓撲的設計- 冗余拓撲能夠解決單點故障問題；冗余拓撲能夠解決單點故障問題；- 冗余拓撲造成冗余拓撲造成 廣播風暴廣播風暴, 多幀復用多幀復用, MAC地址不穩定的問題；地址不穩定的問題；- 站點D發送一個廣播幀；- 廣播幀除源端口以外任意端口泛洪；廣播幀廣播幀廣播風暴廣播風暴- Host X發送廣播幀；- 交換機繼續沒完沒了的更新廣播流量；多幀復用多幀復用- Host X 發送一個單播數據幀給 Router Y；- Router Y 的MAC地址還沒有被每個交換機學習到；- Router Y 接收2份相同的數據幀的拷貝；- Host X 發送一個單播數據幀

2、給 Router Y；- Router Y 的MAC地址還沒有被每個交換機學習到； - Switches A and B 在Port1上學習到Host X的MAC地址；- 到達Router Y的數據幀被泛洪；- Switches A and B 不正確的在Port2上學習到Host X的MAC地址；MAC地址表的不穩定地址表的不穩定- 通過將特定的端口選為 Blocking state，來實現無環的拓撲；- IEEE 802.1D 規定了這一行為；- Cisco采用IEEE802.1D的增強的私有協議生成樹PVST+;采用生成樹采用生成樹STP解決環路解決環路生成樹生成樹STP的操作的操作-

3、每個廣播域選擇一個每個廣播域選擇一個根橋根橋；- 每個非根橋上選擇一個每個非根橋上選擇一個根端口根端口；- 每個段選擇一個每個段選擇一個指定端口指定端口；- 選擇一個選擇一個非指定端口非指定端口；BPDU(Bridge Protocol Data Unit )BPDUs 在交換機之間交換信息在交換機之間交換信息根橋的選舉根橋的選舉STP根橋的選舉根橋的選舉- BPDU (default = sent every 2 seconds)- Root bridge = bridge with the lowest bridge ID- Bridge ID =BridgePriorityMACAddr

4、essPVST+ Extended Bridge IDBridge ID without the extended system IDExtended bridge ID with system IDSystem ID = VLAN手動配置根橋和備份根橋手動配置根橋和備份根橋spanning-tree vlan 1 root primary 強制VLAN 1為交換機的根橋；spanning-tree vlan 2 root secondary 配置交換機為VLAN2的備份根橋；或者spanning-tree vlan # priority priority 修改交換機VLAN優先級 (優先級為

5、4096的倍數).Switch(config)#Switch(config)#Switch(config)#根端口的選舉根端口的選舉STP路徑開銷路徑開銷Link SpeedCost (New IEEE Specification)Cost (Old IEEE Specification)10 Gb/s211 Gb/s41100 Mb/s191010 Mb/s100100根端口的選舉根端口的選舉Fast EthernetEthernetRPNondesignated PortDPDP Switch X 是根橋；是根橋； Switch Y 需要選擇一個根端口；需要選擇一個根端口； Fast Et

6、hernet total cost = 0 + 19. Ethernet total cost = 0 + 100.指定端口的選舉指定端口的選舉Fast EthernetEthernetRPNondesignated PortDPDP Switch X是根橋；是根橋； 根橋上所有的端口都是指定端口，因為他們的路徑開銷為根橋上所有的端口都是指定端口，因為他們的路徑開銷為 0； 因為因為 Ethernet鏈路的路徑開銷為鏈路的路徑開銷為100, switch Y 會阻塞這個端口；會阻塞這個端口；練習：練習：STP操作示例操作示例練習：練習：STP操作示例操作示例每個端口的不同生成樹狀態：每個端口的

7、不同生成樹狀態：STP端口狀態端口狀態STP端口狀態端口狀態 In the blocking state, ports can only receive BPDUs. It may take up to 20 seconds to change from this state； In listening state, switches determine if there are any other paths to the root bridge. the forward delay and lasts for 15 seconds. In the listening state, user

8、 data is not being forwarded and MAC addresses are not being learned； In learning state user data is not forwarded, but MAC addresses are learned from any traffic that is seen. The learning state lasts for 15 seconds and is also called the forward delay； In forwarding state user data is forwarded an

9、d MAC addresses continue to be learned. BPDUs are still processed；PVST+的負載均衡的負載均衡STP特性特性PortFastPortFast 被配置在Access端口上, 而不是在 trunk端口上配置配置STP特性特性PortFastspanning-tree portfastSwitch(config-if)# 在接口上配置PortFast 屬性spanning-tree portfast defaultSwitch(config)# 全局下對所有的 non-trunking 端口啟用PortFast 屬性show run

10、ning-config interface interfaceSwitch# 查看接口配置Portfast的情況或者RSTP (802.1W)Rapid Spanning Tree Protocol(802.1w)802.1W相對于先前的802.1D生成樹來說,在網絡拓撲發生變化的時候提供更快的收斂速度，RSTP中還集成了類似于uplink和backbonefast屬性.RSTP Port StatesRSTP Port RolesWhat Are Edge Ports? Will never have a switch connected to it. Immediately transit

11、ions to forwarding. Functions similarly to PortFast（思科）（思科）. Configured by issuing the spanning-tree portfast command（思科）（思科）. When a edge port receives a BPDU,it generates a TCN.RSTP Link Types*Full-duplex-point-to-point*Half-duplex-shared*通過接口命令修改其默認的鏈路類型通過接口命令修改其默認的鏈路類型spanning-tree link-type ？RS

12、TP Topology Change Mechanism只有當非邊緣端口從阻塞狀態到轉發狀態的時候才會導致拓撲變更.與802.1D不同,連接丟失并不會產生拓撲的變更.換句話說,如果某個狀態進入Blocking狀態將不會導致相應的網橋產生TC BPDU.PVRST Implementation Commands Configuring spanning-tree mode rapid-pvst Verifying show spanning-tree vlan 101 Debugging debug spanning-treeHow to Implement Rapid PVRST(RPVST+

13、) Verifying PVRST Display spanning tree mode is set to PVRST.Switch# show spanning-tree vlan 30 VLAN0030Spanning tree enabled protocol rstpRoot ID Priority 24606Address 00d0.047b.2800This bridge is the rootHello Time 2 sec Max Age 20 sec Forward Delay 15 secBridge ID Priority 24606 (priority 24576 sys-id-ext 30) Address 00