snmp v3安全级别有三种，分别为noAuthNoPriv（不认证也不加密）、authNoPriv（认证但是不加密）、authPriv（既认证又加密）
一、SNMPv3配置代码：
snmp-server contact zhangsan snmp-server location chengdu snmp-server view MIB-2 mib-2 included
1）如果设置安全级别为authPriv
snmp-server group READONLY v3 priv read MIB-2 snmp-server user admin READONLY v3 auth sha mypass123 priv aes 128 myaes123
2）如果设置安全级别为authNoPriv
snmp-server group READONLY v3 auth read MIB-2 snmp-server user admin READONLY v3 auth sha mypass123
3）如果设置安全级别为noAuthNoPriv
snmp-server group READONLY v3 noauth read MIB-2 snmp-server user admin READONLY v3
注意：上述密码设置长度必须大于等于八位，否则在linux上使用snmpwalk命令时，会报错提示
Error: passphrase chosen is below the length requirements of the USM (min=8). snmpwalk: (The supplied password length is too short.) Error generating a key (Ku) from the supplied privacy pass phrase.
其中sha可改成md5，
md5 Use HMAC MD5 algorithm for authentication sha Use HMAC SHA algorithm for authentication
aes可改成：3des，des。
3des Use 168 bit 3DES algorithm for encryption aes Use AES algorithm for encryption des Use 56 bit DES algorithm for encryption
2、snmpwalk获取信息命令
1）安全级别为authPriv时，snmpwalk获取信息方式：
snmpwalk -v 3 -u admin -l authPriv -a sha -A mypass123 -x aes -X myaes123 192.168.0.80 ".1.3.6.1.2.1"
2）安全级别为authNoPriv时，snmpwalk获取信息方式：
snmpwalk -v 3 -u admin -l authNoPriv -a sha -A mypass123 192.168.0.80 ".1.3.6.1.2.1"
3）安全级别为noAuthNoPriv时，snmpwalk获取信息方式：
snmpwalk -v 3 -u admin -l noAuthNoPriv 192.168.0.80 ".1.3.6.1.2.1" snmp-server contact zhangsan snmp-server location chengdu snmp-server view MIB-2 mib-2 included
1）如果设置安全级别为authPriv
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