通过实验理解交换的三个fast特性

 

 

拓扑如上图，Sw1为根交换机，Sw2为备份根交换机，Sw3为接入层交换机。

配置：SW1spanning-tree vlan 1 priority 8192 SW2：spanning-tree vlan 1 priority 16384。

通过在ＳＷ３上查看。

SW3#show span brief

VLAN1
  Spanning tree enabled protocol ieee
  Root ID    Priority    8192
             Address     cc00.0dd4.0000
             Cost        19
             Port        4 (FastEthernet0/3)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32768
             Address     cc02.0dd4.0000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface                                   Designated
Name                 Port ID Prio Cost  Sts Cost  Bridge ID            Port ID
-------------------- ------- ---- ----- --- ----- -------------------- -------
FastEthernet0/2      128.3    128    19 BLK    19 16384 cc01.0dd4.0000 128.3 
FastEthernet0/3      128.4    128    19 FWD     0  8192 cc00.0dd4.0000 128.4 
FastEthernet0/4      128.5    128    19 FWD    19 32768 cc02.0dd4.0000 128.5 

一、Portfast特性。

开启portfast特性的端口又叫快速端口，配置在接口端口上，连接主机或路由器的端口上启用，不能与连接交换机的端口启用。否则或出现环路。默认的情况下接口从接入到Forwarding状态需要经过30s的时间，经过listening、learning状态。验证：

将SW3的Fa0/4关闭，开启degbu spanning-tree event。

SW3(config)#inter f0/4
SW3(config-if)#no shu
SW3(config-if)#
*Mar  1 00:03:00.591: STP: VLAN1 Fa0/4 -> listening
*Mar  1 00:03:03.355: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/4, changed state to up
*Mar  1 00:03:15.611: STP: VLAN1 Fa0/4 -> learning
*Mar  1 00:03:30.631: STP: VLAN1 sent Topology Change Notice on Fa0/3
*Mar  1 00:03:30.635: STP: VLAN1 Fa0/4 -> forwarding

下面开启Portfast特性。

在SW3的Fa0/4接口下:

SW3(config-if)#spanning-tree portfast
%Warning: portfast should only be enabled on ports connected to a single host.
 Connecting hubs, concentrators, switches,  bridges, etc.to this interface
 when portfast is enabled, can cause temporary spanning tree loops.
 Use with CAUTION

%Portfast has been configured on FastEthernet0/4 but will only
 have effect when the interface is in a non-trunking mode.

SW3(config)#inter f0/4

SW3(config-if)#no shu
SW3(config-if)#
*Mar  1 00:05:36.435: STP: VLAN1 Fa0/4 ->jump to forwarding from blocking
*Mar  1 00:05:39.171: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/4, changed state to up

直接进入Forwarding状态。节省30s。

二、uplinkfast。

uplinkfast也叫快速上行端口，在上图所示的拓扑中，接入交换机有两条上行链路如果主链路down，备份链路需要经过30s的时间才能够恢复到Forwarding状态。

验证：

在SW3上将Fa0/3端口。

SW3(config)#int f0/3
SW3(config-if)#shu
SW3(config-if)#
*Mar  1 00:06:47.763: STP: VLAN1 Fa0/3 -> blocking
*Mar  1 00:06:47.763: STP: VLAN1 new root port Fa0/2, cost 38
*Mar  1 00:06:47.835: STP: VLAN1 Fa0/2 -> listening
*Mar  1 00:06:49.687: %LINK-5-CHANGED: Interface FastEthernet0/3, changed state to administratively down
*Mar  1 00:06:49.687: STP: VLAN1 sent Topology Change Notice on Fa0/2
*Mar  1 00:06:50.687: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/3, changed state to down
*Mar  1 00:07:02.855: STP: VLAN1 Fa0/2 -> learning
*Mar  1 00:07:17.875: STP: VLAN1 sent Topology Change Notice on Fa0/2
*Mar  1 00:07:17.879: STP: VLAN1 Fa0/2 -> forwarding

现在启用uplinkfast，SW3上全局启用。uplinkfast一般在接口交换机上启用。核心和汇聚交换机最好不要启用。

SW3(config)#spanning-tree uplinkfast

SW3(config-if)#shut

SW3(config-if)#
*Mar  1 02:53:35.555: STP: VLAN1 Fa0/3 -> blocking
*Mar  1 02:53:35.555: STP: VLAN1 new root port Fa0/2, cost 3038
*Mar  1 02:53:35.555: %SPANTREE_FAST-7-PORT_FWD_UPLINK: VLAN1 FastEthernet0/2 moved to Forwarding (UplinkFast).

备份链路接口Fa0/2直接进入Forwarding状态。节省30s。

注：启用uplinkfast以后，生成树的优先级将变成49152，接口的生成树cost增加3000。

SW3#show span br

VLAN1
  Spanning tree enabled protocol ieee uplinkfast enabled
  Root ID    Priority    8192
             Address     cc00.0dd4.0000
             Cost        3038
             Port        3 (FastEthernet0/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    49152
             Address     cc02.0dd4.0000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface                                   Designated
Name                 Port ID Prio Cost  Sts Cost  Bridge ID            Port ID
-------------------- ------- ---- ----- --- ----- -------------------- -------
FastEthernet0/2      128.3    128  3019 FWD    19 16384 cc01.0dd4.0000 128.3 
FastEthernet0/3      128.4    128  3019 BLK     0  8192 cc00.0dd4.0000 128.4 
FastEthernet0/4      128.5    128  3019 FWD  3038 49152 cc02.0dd4.0000 128.5 

三、Backbonefast（快速骨干）

在上图SW1和SW2的直连链路如果出现故障，那么SW2将无法识别根交换机的位置，SW2就会发送次级BPDU标识自己是根，SW3的阻塞端口收到次级BPDU以后，将会进入老化时间，然后经过listening、learning状态才能到达Forwarding状态，这一过程需要50s的时间。

验证：

将SW2的Fa0/1关闭。

SW2(config)#inter fa0/1

SW2(config-if)#shutdown

SW3#
*Mar  1 01:02:30.967: STP: VLAN1 heard root 16384-cc01.0dd4.0000 on Fa0/2
*Mar  1 01:02:30.967: current Root has  8192-cc00.0dd4.0000
*Mar  1 01:02:32.931: STP: VLAN1 heard root 16384-cc01.0dd4.0000 on Fa0/2
*Mar  1 01:02:32.931: current Root has  8192-cc00.0dd4.0000
*Mar  1 01:02:34.919: STP: VLAN1 heard root 16384-cc01.0dd4.0000 on Fa0/2
*Mar  1 01:02:34.919: current Root has  8192-cc00.0dd4.0000
*Mar  1 01:02:36.907: STP: VLAN1 heard root 16384-cc01.0dd4.0000 on Fa0/2
*Mar  1 01:02:36.907: current Root has  8192-cc00.0dd4.0000
*Mar  1 01:02:38.927: STP: VLAN1 heard root 16384-cc01.0dd4.0000 on Fa0/2
*Mar  1 01:02:38.927: current Root has  8192-cc00.0dd4.0000
*Mar  1 01:02:40.931: STP: VLAN1 heard root 16384-cc01.0dd4.0000 on Fa0/2
*Mar  1 01:02:40.931: current Root has  8192-cc00.0dd4.0000
*Mar  1 01:02:42.923: STP: VLAN1 heard root 16384-cc01.0dd4.0000 on Fa0/2
*Mar  1 01:02:42.923: current Root has  8192-cc00.0dd4.0000
*Mar  1 01:02:44.931: STP: VLAN1 heard root 16384-cc01.0dd4.0000 on Fa0/2
*Mar  1 01:02:44.931: current Root has  8192-cc00.0dd4.0000
*Mar  1 01:02:46.935: STP: VLAN1 heard root 16384-cc01.0dd4.0000 on Fa0/2
*Mar  1 01:02:46.935: current Root has  8192-cc00.0dd4.0000
*Mar  1 01:02:48.927: STP: VLAN1 heard root 16384-cc01.0dd4.0000 on Fa0/2
*Mar  1 01:02:48.927: current Root has  8192-cc00.0dd4.0000
*Mar  1 01:02:48.987: STP: VLAN1 Fa0/2 -> listening
*Mar  1 01:02:50.023: STP: VLAN1 Topology Change rcvd on Fa0/2
*Mar  1 01:02:50.027: STP: VLAN1 sent Topology Change Notice on Fa0/3
*Mar  1 01:03:04.007: STP: VLAN1 Fa0/2 -> learning
*Mar  1 01:03:19.031: STP: VLAN1 sent Topology Change Notice on Fa0/3
*Mar  1 01:03:19.031: STP: VLAN1 Fa0/2 -> forwarding
经过50s的时间。

配置backbonefast，在所有的交换机上全局启用。spanning-tree backbonefast

配置backbonefast特性以后，当SW3收到次级BPDU时，他会向根交换机SW1发送RLQ BPDU（根链路查询BPDU）查询根交换机是否仍然处于激活状态，SW1收到RLQ BPDU以后会发送一个RLQ 确认。SW3收到RLQ的确认以后会将收到次级BPDU的阻塞端口跳过老化时间，直接进入listening状态，节省20s的时间。

SW2(config)#inter fa0/1

SW2(config-if)#shutdown

SW3#

*Mar  1 01:05:11.735: STP: VLAN1 heard root 16384-cc01.0dd4.0000 on Fa0/2
*Mar  1 01:05:11.735: current Root has  8192-cc00.0dd4.0000
*Mar  1 01:05:11.815: STP: VLAN1 Fa0/2 -> listening
*Mar  1 01:05:11.975: STP: VLAN1 Topology Change rcvd on Fa0/2
*Mar  1 01:05:11.975: STP: VLAN1 sent Topology Change Notice on Fa0/3
*Mar  1 01:05:26.835: STP: VLAN1 Fa0/2 -> learning
*Mar  1 01:05:41.859: STP: VLAN1 sent Topology Change Notice on Fa0/3
*Mar  1 01:05:41.859: STP: VLAN1 Fa0/2 -> forwarding