C# SnmpSharpNet UdpTarget.Request 报错 ?报错

C# SnmpSharpNet target.Request 报错 错误：Request has reached maximum retries.
问提描述：用SNMP target.Request 定时（每分）获取交换机端口流量，当获取一段时间后 报错
交换机管理地址PING不通，过十来分钟后交换机又能PING通。不知道问题出在哪？ 是交换机有保护机制还是获取式有问题？ 求解 获取代码如下： 传入IP,端口 161，共同体：public, OID

            public static string getSnmpValByOid(string host, int snmpPort, string community, string oid)
    {
        string val = "";
        OctetString second = new OctetString(community);
        AgentParameters param = new AgentParameters(second);

        param.Version = SnmpVersion.Ver2;

        IpAddress agent = new IpAddress(host);

        UdpTarget target = new UdpTarget((IPAddress)agent, snmpPort, 5000, 1);

        Pdu pdu = new Pdu(PduType.Get);
        pdu.VbList.Add(oid);


        SnmpV2Packet result = (SnmpV2Packet)target.Request(pdu, param);//报错

        if (result != null)
        {

            if (result.Pdu.ErrorStatus == 0)
            {
                val = result.Pdu.VbList[0].Value.ToString();
            }
        }
        target.Close();
        return val;
    }