C# SNMPSharpNet request has reached maximum retries问题

刚刚接触snmp，不太会使用，主要是怎么使用snmp的set方法，确认过ip是正确的，找不到什么原因和解决办法，求指点

            IpAddress agent = new IpAddress("192.168.1.224");
            UdpTarget target = new UdpTarget((IPAddress)agent, 161, 5000, 1);

            //create set pdu
            Pdu set_pdu = new Pdu(PduType.Set);
            set_pdu.VbList.Add(new Oid(".1.3.6.1.4.1.11.2.3.9.4.2.1.1.1.2.0"), new Integer32(1));
            OctetString community = new OctetString("public");
            AgentParameters aparam = new AgentParameters(community);
            aparam.Version = SnmpVersion.Ver2;
            SnmpV2Packet response;
            try
            {
                response = (SnmpV2Packet)target.Request(set_pdu, aparam);
            }
            catch(Exception ex)
            {
                MessageBox.Show(String.Format("Request failed with exception: {0}", ex.Message));
                target.Close();
                return;
            }

            if (response == null)
            {
                Console.WriteLine("error");
            }
            else
            {
                if (response.Pdu.ErrorStatus != 0)
                {
                    MessageBox.Show(String.Format("SNMP agent returned ErrorStatus {0} on index {1}",response.Pdu.ErrorStatus, response.Pdu.ErrorIndex));
                }
                else
                {
                    MessageBox.Show(String.Format("Agent response {0}: {1}",response.Pdu[0].Oid.ToString(), response.Pdu[0].Value.ToString()));
                }
            }