故障现象：
cisco 4006交换机cpu利用率过高，业务时断时续，无法正常进行，交换机日志采集的信息如下：
2007 May 24 03:55:40 %SYS-4-P2_WARN: 1/Host 00:02:fd:06:d0:b0 is flapping between port 1/2 and port 1/1
2007 May 24 03:55:42 %SYS-4-P2_WARN: 1/Host 00:04:de:17:28:20 is flapping between port 1/2 and port 4/45
2007 May 24 03:55:44 %SYS-4-P2_WARN: 1/Host 00:00:0c:07:ac:01 is flapping between port 1/2 and port 4/47
2007 May 24 03:55:45 %SYS-4-P2_WARN: 1/Host 00:05:9a:20:78:20 is flapping between port 1/2 and port 4/47
2007 May 24 03:55:48 %SYS-4-P2_WARN: 1/Host 00:02:fd:06:d0:b0 is flapping between port 1/1 and port 1/2
2007 May 24 03:55:49 %SYS-4-P2_WARN: 1/Host 00:11:25:19:c3:c2 is flapping between port 1/2 and port 4/13
2007 May 24 03:55:53 %PAGP-5-PORTFROMSTP:Port 4/45 left bridge port 4/45
2007 May 24 03:55:54 %SYS-4-P2_WARN: 1/Host 00:06:29:ec:aa:f2 is flapping between port 1/2 and port 4/37
2007 May 24 03:55:54 %SYS-4-P2_WARN: 1/Host 00:10:5c:c5:6a:ca is flapping between port 1/1 and port 4/7
2007 May 24 03:55:54 %SYS-4-P2_WARN: 1/Host 00:09:6b:f5:0f:33 is flapping between port 1/1 and port 4/13
2007 May 24 03:55:54 %SYS-4-P2_WARN: 1/Host 00:10:5c:45:6a:ca is flapping between port 1/2 and port 1/1
2007 May 24 03:55:54 %SYS-4-P2_WARN: 1/Host 00:16:ec:7b:6c:b4 is flapping between port 1/1 and port 1/2
2007 May 24 03:55:55 %SYS-4-P2_WARN: 1/Host 00:10:5c:c5:6a:ca is flapping between port 1/1 and port 4/7
处理步骤：
1、首先重启了两台cisco 4006交换机（其实网络上还连接了两台IBM小型机通过HACMP做了双机，由于双机对共享资源的保护，对备机发出了shutdown命令；正确的做法，应该先关闭一台交换机，或者将备机的hacmp停止后再关闭两台交换机），启动后，cpu利用率下降，业务得以正常进行；
2、接下来，根据报错信息上提到的各个端口检查网络中是否存在环路。经检查，出了两台4006之间有环路外，不存在其他环路，各命令检查结果正常，所用到的命令有：show spantree active,show trunk,show config,show vlan,show port 等。
3、使用端口镜像方式对流经交换机上的数据进行抓包，看是否有可疑的arp包，是否为arp病毒导致网络出现环路。检查结果未发现。使用的命令为:set span;使用的工具为：sniffer。
4、考虑到曾经遇到过cisco STP算法出现bug的情况，决定对两台交换机之间的配置做一个改动，将1/1-2两个光纤端口做成一个channel，然后在做trunk，这样既保持了两台交换机之间的连接冗余，又可以消除环路。使用的命令为：
set port channel 1/1-2 53
set port channel 1/1-2 mode on
两边做完后，通过show port channel查看状态，其中4006-2为notconnect，另一边4006-1为errdisable；在4006－1上执行命令：set port 1/1-2 enable;在使用show port channel查看，两边的状态均为connected；
在其中一台交换机上设置trunk：
set trunk 1/1-2 on 1
使用show trunk命令查看状态正常；
使用show spantree active 查看正常：
4006-2> (enable) show spantree
VLAN 1
Spanning tree enabled
Spanning tree type ieee
Designated Root 00-05-32-db-b0-00
Designated Root Priority 32768
Designated Root Cost 3
Designated Root Port 1/1-2 (agPort 13/1)
Root Max Age 20 sec Hello Time 2 sec Forward Delay 15 sec
Bridge ID MAC ADDR 00-05-32-db-b4-00
Bridge ID Priority 32768
Bridge Max Age 20 sec Hello Time 2 sec Forward Delay 15 sec
Port Vlan Port-State Cost Prio Portfast Channel_id
------------------------ ---- ------------- --------- ---- -------- ----------
1/1-2 1 forwarding 3 32 disabled 769