cisco生成树的三个特性：PortFast.UplinkFast.BackboneFast,用来加速生成树的收敛。
以下面这个拓扑图来讲解
１.PortFast
它能让交换机的端口绕过监听和学习状态，立即进入生成权的转发状态。一般在access接口启用此功能。
现在在switch11交换机上把g0/1划入vlan1,把端口先down，再no shutdown,看下生成树的情况：
*May 5 00:08:35.872: set portid: VLAN0001 Gi0/1: new port id 8002
*May 5 00:08:35.873: STP SW: Gi0/1 new blocking req for 1 vlans
*May 5 00:08:35.874: STP: VLAN0001 Gi0/1 -> listening
SW11(config-if)#
SW11(config-if)#
*May 5 00:08:37.655: %LINK-3-UPDOWN: Interface GigabitEthernet0/1, changed state to up
*May 5 00:08:38.657: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to up
SW11(config-if)#
SW11(config-if)#
*May 5 00:08:50.874: STP SW: Gi0/1 new learning req for 1 vlans
*May 5 00:08:50.875: STP: VLAN0001 Gi0/1 -> learning
SW11(config-if)#
*May 5 00:09:05.877: STP SW: Gi0/1 new forwarding req for 1 vlans
*May 5 00:09:05.878: STP[1]: Generating TC trap for port GigabitEthernet0/1
*May 5 00:09:05.881: STP: VLAN0001 sent Topology Change Notice on Gi0/3
*May 5 00:09:05.882: STP: VLAN0001 Gi0/1 -> forwarding
*May 5 00:08:35.872: set portid: VLAN0001 Gi0/1: new port id 8002没开启PortFast的情况，上面看的很清楚，15秒监听，15秒学习，总共花费30秒才到转发状态。
看下开启了PortFast的情况：
*May 5 00:27:06.018: set portid: VLAN0001 Gi0/1: new port id 8002
*May 5 00:27:06.019: STP SW: Gi0/1 new blocking req for 1 vlans
*May 5 00:27:06.019: STP: VLAN0001 Gi0/1 ->jump to forwarding from blocking
*May 5 00:27:06.020: STP SW: Gi0/1 new forwarding req for 1 vlans
直接跳过监听学习状态，直接到转发状态，节约30秒时间。
PortFast可以在端口下开启，也可以全局开启，在端口下开启只有这个端口启用了；在全局模式下启用，将所有非trunk端口激活portfast特性。
２、UplinkFast
此特性在接入连汇聚层的交换机上配置。加速新的根端口的选择，端口会立即换到转发，不需要经过监听和学习状态，节约３０秒。当切换uplink链路时，接入交换机会以200数据包/秒的速度在新链路上发送伪帧给上游交换机，让上游交换机学习到新链路的信息（也就是刷新CAM表项）。不刷新的话，CAＭ表项还是原来的，数据传输会出错。因为拓扑结构发生了变化。
先看下不开启UplinkFast，需要的收敛时间
sw4中的g1/3是根端口，g0/1上另一条被阻塞的上行链路，现在把g1/3 down掉
SW4(config-if)#
*May 5 01:17:09.767: STP: VLAN0001 new root port Gi0/1, cost 7
*May 5 01:17:09.767: STP SW: Gi0/1 new listening req for 1 vlans
*May 5 01:17:09.767: STP: VLAN0001 Gi0/1 -> listening
*May 5 01:17:09.767: STP[1]: Generating TC trap for port GigabitEthernet1/3
SW4(config-if)#
*May 5 01:17:11.739: %LINK-5-CHANGED: Interface GigabitEthernet1/3, changed state to administratively down
SW4(config-if)#
*May 5 01:17:11.768: STP: VLAN0001 sent Topology Change Notice on Gi0/1
SW4(config-if)#
*May 5 01:17:12.739: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/3, changed state to down
SW4(config-if)#
*May 5 01:17:24.767: STP SW: Gi0/1 new learning req for 1 vlans
*May 5 01:17:24.768: STP: VLAN0001 Gi0/1 -> learning
SW4(config-if)#
*May 5 01:17:28.651: STP: VLAN0001 sent Topology Change Notice on Gi0/1
*May 5 01:17:28.651: STP: VLAN0001 Topology Change rcvd on Gi0/3
*May 5 01:17:28.652: STP: VLAN0001 Topology Change rcvd on Gi1/0
SW4(config-if)#
*May 5 01:17:39.769: STP SW: Gi0/1 new forwarding req for 1 vlans
*May 5 01:17:39.769: STP[1]: Generating TC trap for port GigabitEthernet0/1
*May 5 01:17:39.771: STP: VLAN0001 sent Topology Change Notice on Gi0/1
*May 5 01:17:39.771: STP: VLAN0001 Gi0/1 -> forwarding
SW4(config-if)#
g0/1经过监听学习到达转发状态，用时３０秒
再看下开启UplinkFast时的情况
*May 5 01:22:49.189: STP: VLAN0001 new root port Gi0/1, cost 3007
*May 5 01:22:49.190: %SPANTREE_FAST-7-PORT_FWD_UPLINK: VLAN0001 GigabitEthernet0/1 moved to Forwarding (UplinkFast).
*May 5 01:22:49.190: STP SW: Gi0/1 new forwarding req for 1 vlans
g0/1直接到转发状态，节约30秒。
３、BackboneFast
根桥和备份根桥之间的连接出现问题，BackboneFast特性可以让下面的交换机端口节约２０秒(max age)收敛时间，但仍需要３０秒(监听和学习)时间来收敛。
如上图所示，sw2为主根桥，sw3为备份根桥，sw4的g1/3为RP，G0/1为NDP端口。
SW4#show spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 1
Address 5000.0002.0000
Cost 3004
Port 8 (GigabitEthernet1/3)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 49153 (priority 49152 sys-id-ext 1)
Address 5000.0004.0000
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Uplinkfast enabled
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0 Desg FWD 3004 128.1 P2p
Gi0/1 Altn BLK 3004 128.2 P2p
Gi0/2 Desg FWD 3004 128.3 P2p
Gi0/3 Desg FWD 3004 128.4 P2p
Gi1/0 Desg FWD 3004 128.5 P2p
Gi1/1 Desg FWD 3004 128.6 P2p
Gi1/2 Desg FWD 3004 128.7 P2p
Gi1/3 Root FWD 3004 128.8 P2p
当主根桥sw2和备份根桥sw3之间的链路发生问题时（sw3的g1/1和g1/2口也就是以太网信道port 23口down时），sw3收不到bpdu，会认为到根桥的链接有问题，就认为自己是根桥从g0/1口发送bpdu给sw4。sw4从g0/1端口收到了bpdu(是一个次级bpdu),如果不开启backbonefast,则g0/1端口要等待最大时间２０（max age）秒超时才进入监听状态，开启backbonefast后，g0/1直接进入监听状态，节约这20秒时间，然后进入学习状态，最后到转发（经过30秒）。
不开启backbonefast的情况下，看sw4的情况：
*May 8 00:44:54.195: STP: VLAN0001 heard root 4097-5000.0003.0000 on Gi0/1
SW4#
*May 8 00:44:56.178: STP: VLAN0001 heard root 4097-5000.0003.0000 on Gi0/1
SW4#
*May 8 00:44:58.198: STP: VLAN0001 heard root 4097-5000.0003.0000 on Gi0/1
SW4#
*May 8 00:45:00.170: STP: VLAN0001 heard root 4097-5000.0003.0000 on Gi0/1
SW4#
*May 8 00:45:02.203: STP: VLAN0001 heard root 4097-5000.0003.0000 on Gi0/1
SW4#
*May 8 00:45:04.172: STP: VLAN0001 heard root 4097-5000.0003.0000 on Gi0/1
SW4#
*May 8 00:45:06.171: STP: VLAN0001 heard root 4097-5000.0003.0000 on Gi0/1
SW4#
*May 8 00:45:08.161: STP: VLAN0001 heard root 4097-5000.0003.0000 on Gi0/1
SW4#
*May 8 00:45:10.152: STP: VLAN0001 heard root 4097-5000.0003.0000 on Gi0/1
SW4#
*May 8 00:45:12.147: STP: VLAN0001 heard root 4097-5000.0003.0000 on Gi0/1
*May 8 00:45:12.195: STP SW: Gi0/1 new listening req for 1 vlans
*May 8 00:45:12.196: STP: VLAN0001 Gi0/1 -> listening
SW4#
*May 8 00:45:13.277: STP: VLAN0001 Topology Change rcvd on Gi0/1
*May 8 00:45:13.280: STP: VLAN0001 sent Topology Change Notice on Gi1/3
SW4#
*May 8 00:45:27.198: STP SW: Gi0/1 new learning req for 1 vlans
*May 8 00:45:27.199: STP: VLAN0001 Gi0/1 -> learning
SW4#
*May 8 00:45:42.200: STP SW: Gi0/1 new forwarding req for 1 vlans
*May 8 00:45:42.201: STP[1]: Generating TC trap for port GigabitEthernet0/1
*May 8 00:45:42.204: STP: VLAN0001 sent Topology Change Notice on Gi1/3
*May 8 00:45:42.205: STP: VLAN0001 Gi0/1 -> forwarding
开启backbonefast时，查看sw4的情况：
SW4(config)#
*May 8 00:58:47.939: STP: VLAN0001 heard root 4097-5000.0003.0000 on Gi0/1
*May 8 00:58:47.944: STP SW: Gi0/1 new listening req for 1 vlans
*May 8 00:58:47.944: STP: VLAN0001 Gi0/1 -> listening
*May 8 00:58:48.446: STP: VLAN0001 Topology Change rcvd on Gi0/1
*May 8 00:58:48.448: STP: VLAN0001 sent Topology Change Notice on Gi1/3
SW4(config)#
*May 8 00:59:02.945: STP SW: Gi0/1 new learning req for 1 vlans
*May 8 00:59:02.945: STP: VLAN0001 Gi0/1 -> learning
SW4(config)#
*May 8 00:59:17.947: STP SW: Gi0/1 new forwarding req for 1 vlans
*May 8 00:59:17.947: STP[1]: Generating TC trap for port GigabitEthernet0/1
*May 8 00:59:17.950: STP: VLAN0001 sent Topology Change Notice on Gi1/3
*May 8 00:59:17.950: STP: VLAN0001 Gi0/1 -> forwarding
