背景：两台Cisco3750做双机，Cisco3750-01硬件故障，罢工了。新买的二手交换机与之前的IOS不一致，故需要更换IOS。
准备工作：网线、console线、tftp工具
操作步骤：
一、从Cisco3750-02上下载以前的IOS
1、在交换机接口上配置一个接口地址；
SW-A-02(3750-2)(config)#int g1/0/23
SW-A-02(3750-2)(config-if)#no switchport
SW-A-02(3750-2)(config-if)#ip address 192.168.100.100 255.255.255.0
2、笔记本用网线连接到交换机；
笔记本使用ip地址192.168.100.101，开启tftp工具（3CDaemon）
3、下载IOS；
（1）笔记本与交换机连通性测试
SW-A-02(3750-2)#ping 192.168.100.101
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.100.101, timeout is 2 seconds:
.!!!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 1/1/1 ms
（2）查看Flash信息
SW-A-02(3750-2)#show flash
Directory of flash:/
2 -rwx 2116 Jan 10 2013 18:53:29 +08:00 vlan.dat
3 drwx 192 Mar 1 1993 08:19:14 +08:00 c3750-i5-mz.122-20.SE4
89 -rwx 16406 Sep 20 2014 17:05:31 +08:00 config.text
90 -rwx 3110 Mar 13 1993 12:40:51 +08:00 startup-configw
91 -rwx 27 Sep 20 2014 17:05:31 +08:00 private-config.text
15998976 bytes total (7386112 bytes free)
SW-A-02(3750-2)#cd c3750-i5-mz.122-20.SE4
SW-A-02(3750-2)#dir
Directory of flash:/c3750-i5-mz.122-20.SE4/
4 drwx 2688 Mar 1 1993 08:16:56 +08:00 html
86 -rwx 5044988 Mar 1 1993 08:19:14 +08:00 c3750-i5-mz.122-20.SE4.bin
87 -rwx 494 Mar 1 1993 08:19:14 +08:00 info
15998976 bytes total (7386112 bytes free)
SW-A-02(3750-2)#pwd
flash:/c3750-i5-mz.122-20.SE4/
（3）将IOS从Flash复制到tftp
SW-A-02(3750-2)#copy flash: tftp:
Source filename []? c3750-i5-mz.122-20.SE4
Address or name of remote host []? 192.168.100.101
Destination filename [c3750-i5-mz.122-20.SE4]?
%Error reading flash:/c3750-i5-mz.122-20.SE4 (Is a directory)
复制失败，无法复制文件夹
SW-A-02(3750-2)#cd c3750-i5-mz.122-20.SE4
SW-A-02(3750-2)#dir
Directory of flash:/c3750-i5-mz.122-20.SE4/
4 drwx 2688 Mar 1 1993 08:16:56 +08:00 html
86 -rwx 5044988 Mar 1 1993 08:19:14 +08:00 c3750-i5-mz.122-20.SE4.bin
87 -rwx 494 Mar 1 1993 08:19:14 +08:00 info
15998976 bytes total (7386112 bytes free)
SW-A-02(3750-2)#copy flash: tftp:
Source filename [/c3750-i5-mz.122-20.SE4/c3750-i5-mz.122-20.SE4.bin]?
Address or name of remote host []? 192.168.100.101
Destination filename [/c3750-i5-mz.122-20.SE4/c3750-i5-mz.122-20.SE4.bin]?
.....
%Error opening tftp:// 192.168.100.101//c3750-i5-mz.122-20.SE4/c3750-i5-mz.122-20.SE4.bin (Timed out)
复制ios文件仍然失败，关闭掉笔记本上的防火墙后，复制正常。
SW-A-02(3750-2)#copy flash: tftp:
Source filename [/c3750-i5-mz.122-20.SE4/c3750-i5-mz.122-20.SE4.bin]?
Address or name of remote host []? 192.168.100.101
Destination filename [/c3750-i5-mz.122-20.SE4/c3750-i5-mz.122-20.SE4.bin]?
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
5044988 bytes copied in 13.916 secs (362531 bytes/sec)
二、更新新交换机上的IOS
1、为了以防万一，先将该交换机的IOS备份（方法与上述方式相同）；
Switch#show flash
Directory of flash:/
3 -rwx 11518717 Mar 4 2010 07:05:23 +00:00 c3750-ipservices-mz.122-50.SE3.bin
4 -rwx 2072 Mar 1 1993 00:03:07 +00:00 multiple-fs
15998976 bytes total (4476416 bytes free)
Switch#show boot
BOOT path-list : flash:c3750-ipservices-mz.122-50.SE3.bin
Config file : flash:/config.text
Private Config file : flash:/private-config.text
Enable Break : no
Manual Boot : no
HELPER path-list :
Auto upgrade : yes
Auto upgrade path :
Timeout for Config
Download: 0 seconds
Config Download
via DHCP: disabled (next boot: disabled)
Switch#copy flash: tftp:
Source filename []? c3750-ipservices-mz.122-50.SE3.bin
Address or name of remote host []? 192.168.100.101
Destination filename [c3750-ipservices-mz.122-50.SE3.bin]?
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
11518717 bytes copied in 31.868 secs (361451 bytes/sec)
2、删除以前的IOS
Switch#delete flash:c3750-ipservices-mz.122-50.SE3.bin
Delete filename [c3750-ipservices-mz.122-50.SE3.bin]?
Delete flash:c3750-ipservices-mz.122-50.SE3.bin? [confirm]
Switch#show flash
Directory of flash:/
4 -rwx 2072 Mar 1 1993 00:03:07 +00:00 multiple-fs
15998976 bytes total (15995392 bytes free)
3、上传新的IOS
Switch#copy tftp: flash:
Address or name of remote host []? 192.168.100.101
Source filename []? c3750-i5-mz.122-20.SE4/c3750-i5-mz.122-20.SE4.bin
Destination filename [c3750-i5-mz.122-20.SE4.bin]?
Accessing tftp://192.168.100.101/c3750-i5-mz.122-20.SE4/c3750-i5-mz.122-20.SE4.bin...
Loading c3750-i5-mz.122-20.SE4/c3750-i5-mz.122-20.SE4.bin from 192.168.100.101 (via GigabitEthernet1/0/24): !!!!!!!!!!!!!!!!!!!!
[OK - 5044988 bytes]
5044988 bytes copied in 114.010 secs (44250 bytes/sec)
4、更改IOS启动顺序
Switch#show boot
BOOT path-list : flash:c3750-ipservices-mz.122-50.SE3.bin
Config file : flash:/config.text
Private Config file : flash:/private-config.text
Enable Break : no
Manual Boot : no
HELPER path-list :
Auto upgrade : yes
Auto upgrade path :
Timeout for Config
Download: 0 seconds
Config Download
via DHCP: disabled (next boot: disabled)
Switch#config t
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)#boot system flash:c3750-i5-mz.122-20.SE4.bin
Switch#show boot
*Mar 1 00:17:10.775: %SYS-5-CONFIG_I: Configured from console by console
BOOT path-list : flash:c3750-i5-mz.122-20.SE4.bin
Config file : flash:/config.text
Private Config file : flash:/private-config.text
Enable Break : no
Manual Boot : no
HELPER path-list :
Auto upgrade : yes
Auto upgrade path :
Timeout for Config
Download: 0 seconds
Config Download
via DHCP: disabled (next boot: disabled)
5、保存后重启交换机
Switch#write
Building configuration...
[OK]
Switch#reload
Proceed with reload? [confirm]