Network Management Using SNMP

1. SNMP Overview

The Simple Network Management Protocol (SNMP) is an applications-layer protocol used to exchange the management information between network devices (e.g., routers). By using SNMP, network administrators can more easily manage network performance, find and solve network problems. The SNMP is a member of the TCP/IP protocol suite, it uses the UDP to exchange messages between a management Client and an Agent, residing in a network node.

There are two versions of SNMP: Version 1 and Version 2. ZyXEL supports SNMPv1. Most of the changes introduced in Version 2 increase SNMP's security capabilities. SNMP encompasses three main areas:

1. A small set of management operations.
2. Definitions of management variables.
3. Data representation.

The operations allowed are: Get, GetNext, Set, and Trap. These functions operates on variables that exist in network nodes. Examples of variables include statistic counters, node port status, and so on. All of the SNMP management functions are carried out through these simple operations. No action operations are available, but these can be simulated by the setting of flag variables. For example, to reset a node, a counter variable named 'time to reset' could be set to a value, causing the node to reset after the time had elapsed.

SNMP variables are defined using the OSI Abstract Syntax Notation One (ASN.1). ASN.1 specifies how a variable is encoded in a transmitted data frame; it is very powerful because the encoded data is self-defining. For example, the encoding of a text string includes an indication that the data unit is a string, along with its length and value. ASN.1 is a flexible way of defining protocols, especially for network management protocols where nodes may support different sets of manageable variables.

The net of variables that each node supports is called the Management Information Base (MIB). The MIB is made up of several parts, including the Standard MIB, specified as part of SNMP, and Enterprise Specific MIB, which are defined by different manufacturer for hardware specific management.

The current Internet-standard MIB, MIB-II, is defined in RFC 1213 and contains 171 objects. These objects are grouped by protocol (including TCP, IP, UDP, SNMP, and other categories, including 'system' and 'interface.'

The Internet Management Model is as shown in figure 1. Interactions between the NMS and managed devices can be any of four different types of commands:

1. Reads

   Read is used to monitor the managed devices, NMSs read variables that are maintained by the devices.

2. Writes

   Write is used to control the managed devices, NMSs write variables that are stored in the managed devices.

3. Traversal operations

   NMSs use these operations to determine which variables a managed device supports and to sequentially gather information from variable tables (such as IP routing table) in managed devices.

4. Traps

The managed devices to asynchronously report certain events to NMSs use trap.

2. SNMPv1 Operations

SNMP itself is a simple request/response protocol. 4 SNMPv1 operations are defined as below.

- Get
  Allows the NMS to retrieve an object variable from the agent.
- GetNext
  Allows the NMS to retrieve the next object variable from a table or list within an agent. In SNMPv1, when a NMS wants to retrieve all elements of a table from an agent, it initiates a Get operation, followed by a series of GetNext operations.
- Set
  Allows the NMS to set values for object variables within an agent.
- Trap
  Used by the agent to inform the NMS of some events.

The SNMPv1 messages contains two part. The first part contains a version and a community name. The second part contains the actual SNMP protocol data unit (PDU) specifying the operation to be performed (Get, Set, and so on) and the object values involved in the operation. The following figure shows the SNMPv1 message format.

The SNMP PDU contains the following fields:

- PDU type Specifies the type of PDU.
- Request ID Associates requests with responses.
- Error status Indicates an error and an error type.
- Error index Associates the error with a particular object variable.
- Variable-bindings Associates particular object with their value.

2. ZyXEL SNMP Implementation

ZyXEL currently includes SNMP support in some Prestige routers. It is implemented based on the SNMPv1, so it will be able to communicate with SNMPv1 NMSs. Further, users can also add ZyXEL's private MIB in the NMS to monitor and control additional system variables. The ZyXEL's private MIB tree is shown in figure 3. For SNMPv1 operation, ZyXEL permits one community string so that the router can belong to only one community and allows trap messages to be sent to only one NMS manager.

Some traps are sent to the SNMP manager when anyone of the following events happens:

1. coldStart (defined in RFC-1215) :

If the machine coldstarts, the trap will be sent after booting.

2. warmStart (defined in RFC-1215) :

If the machine warmstarts, the trap will be sent after booting.

3. linkDown (defined in RFC-1215) :

If any link of IDSL or WAN is down, the trap will be sent with the port number . The port number is its interface index under the interface group.

4. linkUp (defined in RFC-1215) :

If any link of IDSL or WAN is up, the trap will be sent with the port number . The port number is its interface index under the interface group.

5. authenticationFailure (defined in RFC-1215) :

> When receiving any SNMP get or set requirement with wrong community, this trap is sent to the manager.

6. whyReboot (defined in ZYXEL-MIB) :

> When the system is going to restart (warmstart), the trap will be sent with the reason of restart before rebooting.

> (i) For intentional reboot :

> In some cases (download new files, CI command "sys reboot", ...), reboot is done intentionally. And traps with the message "System reboot by user !" will be sent.

> (ii) For fatal error :

> System has to reboot for some fatal errors. And traps with the message of the fatal code will be sent.

- Downloading ZyXEL's private MIB

3. Configure the Prestige for SNMP

The SNMP related settings in Prestige are configured in menu 22, SNMP Configuration. The following steps describe a simple setup procedure for configuring all SNMP settings.

`Menu 22 - SNMP Configuration` `SNMP:` `Get Community= public` `Set Community= public` `Trusted Host= 192.168.1.33` `Trap:` `Community= public` `Destination= 192.168.1.33` `Press ENTER to Confirm or ESC to Cancel:`

Key Settings:

Option | Descriptions
Get Community | Enter the correct Get Community. This Get Community must match the 'Get-' and 'GetNext' community requested from the NMS. The default is 'public'.
Set Community | Enter the correct Set Community. This Set Community must match the 'Set-community requested from the NMS. The default is 'public'.
Trusted Host | Enter the IP address of the NMS. The Prestige will only respond to SNMP messages coming from this IP address. If 0.0.0.0 is entered, the Prestige will respond to all NMS managers.
Trap Community | Enter the community name in each sent trap to the NMS. This Trap Community must match what the NMS is expecting. The default is 'public'.
Trap Destination | Enter the IP address of the NMS that you wish to send the traps to. If 0.0.0.0 is entered, the Prestige will not send trap any NMS manager.

All contents copyright © 2003 ZyXEL Communications Corporation.