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+
+<head>
+<meta http-equiv="Content-Language" content="en-us">
+<meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
+<title>Content</title>
+<base target="_self">
+</head>
+
+<body background="../../images/bg.gif" topmargin="0" leftmargin="0" marginheight="0" marginwidth="0" onLoad="window.focus();" link="#808080" vlink="#808080" alink="#808080">
+
+<table border="0" cellpadding="0" cellspacing="0" width="100%">
+ <tr>
+ <td bgcolor="#336666" width="18" valign="top">
+ <img border="0" src="../../images/content_lines.gif" width="16" height="25">
+ <img border="0" src="../../images/transdot.gif" width="2" height="1"></td>
+ <td bgcolor="#336666"><b><font face="Arial" size="2" color="#FFFFFF">6.2</b></font></td>
+ <td bgcolor="#336666"><img border="0" src="../../images/transdot.gif" width="10" height="1"></td>
+ <td bgcolor="#336666" width="100%"><strong><font size="2" color="#FFFFFF">
+ <span style="font-family: Arial">Network Management</span></font></strong></td>
+ <td width="9" bgcolor="#336666"> </td>
+ </tr>
+ <tr>
+ <td bgcolor="#669999" height="25" width="18"> </td>
+ <td bgcolor="#669999" height="25"><b>
+ <font face="Arial" size="2" color="#FFFFFF">6.2.6</font></b></td>
+ <td bgcolor="#669999"><img border="0" src="../../images/transdot.gif" width="10" height="1"></td>
+ <td bgcolor="#669999" height="25" width="100%"><strong>
+ <font face="Arial" size="2" color="#FFFFFF">SNMP protocol</font></strong></td>
+ <td bgcolor="#669999" height="25" width="9"> </td>
+ </tr></table>
+
+<table border="0" cellpadding="0" cellspacing="0" width="95%" bordercolor="#111111">
+ <tr>
+ <td width="15"></td>
+ <td>
+
+ <font face="Arial" size="2">The agent is a software function embedded
+ in most networked devices, such as routers, switches, managed hubs,
+ printers, and servers.
+ <img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12">
+ It is responsible for processing SNMP requests from the manager. It is
+ also responsible for the execution of routines that maintain variables
+ as defined in the various supported MIBs.</font><p>
+
+ <font face="Arial" size="2"> Interaction between the manager and the
+ agent is facilitated by the Simple Network Management Protocol (SNMP).
+ The term simple comes from the restricted number of message types that
+ are part of the initial protocol specification. The strategy was
+ designed to make it easier for developers to build management
+ capabilities into network devices. The initial protocol specification
+ is referred to as SNMPv1 (version 1).</font></p>
+ <p>
+
+ <font size="2" face="Arial">There are three types of SNMP messages
+ issued on behalf of an NMS. They are GetRequest, GetNextRequest and
+ SetRequest.
+ <img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12">
+ All three messages are acknowledged by the agent in the form of a
+ GetResponse message. An agent may issue a Trap message in
+ response to an event that affects the MIB and the underlying
+ resources.</font></p>
+ <p>
+
+ <font face="Arial" size="2">The development of SNMPv2c addressed
+ limitations in SNMPv1. The most noticeable enhancements were the
+ introduction of the GetBulkRequest message type and the addition of
+ 64-bit counters to the MIB. Retrieving information with GetRequest and
+ GetNextRequest was an inefficient method of collecting information.
+ Only one variable at a time could be solicited with SNMPv1. The
+ GetBulkRequest addresses this weakness by receiving more information
+ with a single request. Secondly, the 64-bit counters addressed the
+ issue of counters rolling over too quickly, especially with higher
+ speed links like Gigabit Ethernet.</font></p>
+ <p>
+
+ <font face="Arial" size="2">The management entity is also referred to
+ as the manager or network management station (NMS).
+ <img border="0" src="../../images/3.gif" align="absmiddle" width="12" height="12">
+ It is responsible for soliciting information from the agent. The
+ solicitations are based on very specific requests. The manager
+ processes the retrieved information in a number of ways. The retrieved
+ information can be logged for later analysis, displayed using a
+ graphing utility, or compared with preconfigured values to test if a
+ particular condition has been met.</font></p>
+ <p>
+
+ <font face="Arial" size="2"> Not all manager functions are based on
+ data retrieval. There is also the ability to issue changes of a value
+ in the managed device. This feature enables an administrator to
+ configure a managed device using SNMP.</font></p>
+ <p>
+
+ <font face="Arial" size="2">The interaction between the manager and
+ the managed device does introduce traffic to the network. Caution
+ should be taken when introducing managers on to the network.
+ Aggressive monitoring strategies can negatively affect network
+ performance. Bandwidth utilizations will go up, which may be an issue
+ for WAN environments. Also, monitoring has a performance impact on the
+ devices being monitored, since they are required to process the
+ manager requests. This
+ processing should not take precedence over production services.</font></p>
+ <p>
+
+ <font face="Arial" size="2"> A general rule is that a minimum amount
+ of information should be polled as infrequently as possible.
+ Determine which devices and links are most critical and
+ what type of data is required.</font></p>
+ <p>
+
+ <font face="Arial" size="2"> SNMP uses UDP as a transport protocol.
+ Since UDP is connectionless and unreliable, it is possible for SNMP to
+ lose messages. SNMP itself has no provision for guarantee of delivery,
+ so it is up to the application using SNMP to cope with lost messages.</font></p>
+ <p>
+
+ <font face="Arial" size="2">Each SNMP message contains a cleartext
+ string, called a community string. The community string is used like a
+ password to restrict access to managed devices.
+ <img border="0" src="../../images/4.gif" align="absmiddle" width="12" height="12"> SNMPv3
+
+ has addressed the security concerns raised by tranmitting
+ the community string in cleartext.</font></p>
+ <p>
+
+ <font face="Arial" size="2"> An example of what the SNMPv2c message
+ looks like is illustrated in Figure
+ <img border="0" src="../../images/5.gif" align="absmiddle" width="12" height="12">. A
+ detailed presentation of the protocol can be found in the Internet
+ standard RFC1905.</font></p>
+ <p>
+
+ <font face="Arial" size="2"> The fact that the community string is
+ cleartext is no surprise to anyone who has studied the Internet
+ Protocol (IP) protocol suite. All fields specified in the protocol
+ suite are cleartext, except for security authentication and encryption
+ specifications.</font></p>
+ <p>
+
+ <font face="Arial" size="2"> The community string was essentially a security placeholder until the
+ SNMPv2 working group could ratify security mechanisms. The efforts
+ were referred to the SNMPv3 working group. All SNMP-based management applications need to be configured to use the
+ appropriate community strings. Some organizations
+ frequently change the community string values to reduce the risk of
+ malicious activity from the unauthorized use of the SNMP service.</font></p>
+ <p>
+
+ <font face="Arial" size="2"> In spite of the weakness associated with
+ community-based authentication, management strategies are still based
+ on SNMPv1. Cisco devices do support SNMPv3 message types and
+ the increased security capabilities, but most management
+ software applications do not support SNMPv3.
+ <img border="0" src="../../images/6.gif" align="absmiddle" width="12" height="12"></font></p>
+ <p>
+
+ <font face="Arial" size="2"> SNMPv3 supports the concurrent existence
+ of multiple security models.
+ <img border="0" src="../../images/7.gif" align="absmiddle" width="12" height="12"></font></p>
+ <p>
+
+ <font face="Arial" size="2"> </font>
+ </td>
+ </tr>
+ </table>
+
+</body>
+
+</html>
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