交换机端口“假死”的现象，在交换机不重启的状态下将该端口“拯救”回来的方法。
拯救步骤1：查看日志/端口的状态
登录进入交换机后，执行show log，会看到如下的提示：
21w6d: %ETHCNTR-3-LOOP_BACK_DETECTED: Keepalive packet loop-back detected on FastEthernet0/20.
21w6d: %PM-4-ERR_DISABLE: loopback error detected on Fa0/20, putting Fa0/20 in err-disable state
以上信息就明确表示由于检测到第20端口出现了环路，所以将该端口置于了err-disable状态。
查看端口的状态
Switch# show inter fa0/20 status
Port Name Status Vlan Duplex Speed Type
Fa0/20 link to databackup err-disabled 562 auto auto 10/100BaseTX
这条信息更加明确的表示了该端口处于err-disabled状态。
该端口是被置于了错误的状态。
拯救步骤2：将端口从错误状态中恢复回来
进入交换机全局配置模式，执行errdisable recovery cause ?，会看到如下信息：
Switch(config)#errdisable recovery cause ?
all Enable timer to recover from all causes
bpduguard Enable timer to recover from BPDU Guard error disable state
channel-misconfig Enable timer to recover from channel misconfig disable state
dhcp-rate-limit Enable timer to recover from dhcp-rate-limit error disable state
dtp-flap Enable timer to recover from dtp-flap error disable state
gbic-invalid Enable timer to recover from invalid GBIC error disable state
l2ptguard Enable timer to recover from l2protocol-tunnel error disable state
link-flap Enable timer to recover from link-flap error disable state
loopback Enable timer to recover from loopback detected disable state
pagp-flap Enable timer to recover from pagp-flap error disable state
psecure-violation Enable timer to recover from psecure violation disable state
security-violation Enable timer to recover from 802.1x violation disable state
udld Enable timer to recover from udld error disable state
unicast-flood Enable timer to recover from unicast flood disable state
vmps Enable timer to recover from vmps shutdown error disable state
从列出的选项中，可以看出，有非常多的原因会引起端口被置于错误状态，由于我们明确的知道这台交换机上的端口是由于环路问题而被置于错误状态的，所以就可以直接键入命令：
Switch(config)#errdisable recovery cause loopback
拯救步骤3：显示被置于错误状态端口的恢复情况
Switch# show errdisable recovery
ErrDisable Reason Timer Status
----------------- --------------
udld Disabled
bpduguard Disabled
security-violatio Disabled
channel-misconfig Disabled
vmps Disabled
pagp-flap Disabled
dtp-flap Disabled
link-flap Disabled
gbic-invalid Disabled
l2ptguard Disabled
psecure-violation Disabled
gbic-invalid Disabled
dhcp-rate-limit Disabled
unicast-flood Disabled
loopback Enabled
Timer interval: 300 seconds
Interfaces that will be enabled at the next timeout:
Interface Errdisable reason Time left(sec)
--------- ----------------- --------------
Fa0/8 loopback 276
Fa0/17 loopback 267
Fa0/20 loopback 250
从以上显示的信息可以看出，这台交换机有三个端口（Fa0/8、Fa0/17、Fa0/20）会分别在276、267、250秒之后恢复为正常的状态，实际情况也是这样，等了几分钟以后，我们找了一台笔记本电脑，分别接到这几个端口上试了一下，端口都可以正常工作了。这下总算在不重交换机的情况下，将几个处于“假死”状态的端口“拯救”了回来。
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为了让交换机出现此假死故障后能自动恢复，我们也有相应帮忙来解决
用于cisco交换机自动恢复端口假死的故障
在特权模式下配置如下：
errdisable recovery cause udld
errdisable recovery cause bpduguard
errdisable recovery cause security-violation
errdisable recovery cause channel-misconfig
errdisable recovery cause pagp-flap
errdisable recovery cause dtp-flap
errdisable recovery cause link-flap
errdisable recovery cause sfp-config-mismatch
errdisable recovery cause gbic-invalid
errdisable recovery cause l2ptguard
errdisable recovery cause psecure-violation
errdisable recovery cause dhcp-rate-limit
errdisable recovery cause unicast-flood
errdisable recovery cause vmps
errdisable recovery cause storm-control
errdisable recovery cause inline-power
errdisable recovery cause arp-inspection
errdisable recovery cause loopback
以上这些都是导致端口假死的条件，配置上述可以
Link-flap error
Link flap means that the interface continually goes up and down. The interface is put into the errdisabled state if it flaps more than five times in 10 seconds. The common cause of link flap is a Layer 1 issue such as a bad cable, duplex mismatch, or bad Gigabit Interface Converter (GBIC) card. Look at the console messages or the messages that were sent to the syslog server that state the reason for the port shutdown.
%PM-4-ERR_DISABLE: link-flap error detected on Gi4/1, putting Gi4/
1 in err-disable state
Issue this command in order to view the flap values:
cat6knative#show errdisable flap-values
!--- Refer to show errdisable flap-values for more information on the command.
ErrDisable Reason Flaps Time (sec)
----------------- ------ ----------
pagp-flap 3 30
dtp-flap 3 30
link-flap 5 10
让端口在出现假死后自动恢复；