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+ + 6.2 + Network Management 
  + 6.2.6 + SNMP protocol 
+ + + + + + +
+ + The agent is a software function embedded + in most networked devices, such as routers, switches, managed hubs, + printers, and servers. + + 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.

+ + 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).

+

+ + There are three types of SNMP messages + issued on behalf of an NMS. They are GetRequest, GetNextRequest and + SetRequest. + + 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.

+

+ + 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.

+

+ + The management entity is also referred to + as the manager or network management station (NMS). + + 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.

+

+ + 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.

+

+ + 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.

+

+ + 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.

+

+ + 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.

+

+ + 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. + SNMPv3 + + has addressed the security concerns raised by tranmitting + the community string in cleartext.

+

+ + An example of what the SNMPv2c message + looks like is illustrated in Figure + . A + detailed presentation of the protocol can be found in the Internet + standard RFC1905.

+

+ + 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.

+

+ + 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.

+

+ + 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. +

+

+ + SNMPv3 supports the concurrent existence + of multiple security models. +

+

+ +     +

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