Configuring SNMP

This chapter describes how to configure the Simple Network Management Protocol (SNMP) on your mobile node.

Note For complete syntax and usage information for the commands used in this chapter, refer to the Cisco IOS Command Reference for this release and to the Cisco IOS Configuration Fundamentals Command Reference for this release.

Understanding SNMP

SNMP is an application-layer protocol that provides a message format for communication between SNMP managers and agents. The SNMP manager can be part of a network management system (NMS) such as CiscoWorks. The agent and management information base (MIB) reside on the network device. To configure SNMP, you define the relationship between the manager and the agent.

The SNMP agent contains MIB variables whose values the SNMP manager can request or change. A manager can get a value from an agent or store a value into the agent. The agent gathers data from the MIB, the repository for information about device parameters and network data. The agent can also respond to a manager's requests to get or set data.

An agent can send unsolicited traps to the manager. Traps are messages alerting the SNMP manager to a condition on the network. Traps can mean improper user authentication, restarts, link status (up or down), MAC address tracking, closing of a TCP connection, loss of connection to a neighbor, or other significant events.

SNMP Versions

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SNMPv3 provides secure access to devices by a combination of authenticating and encrypting packets over the network, defined in RFC 2273, RFC 2274, and RFC 2275.

Both SNMPv1 and SNMPv2C use a community-based form of security. The community of managers able to access the agent's MIB is defined by an IP address access control list and password.

SNMPv2C replaces the Party-based Administrative and Security Framework of SNMPv2Classic with the Community-based Administrative Framework of SNMPv2C while retaining the bulk retrieval and improved error handling of SNMPv2Classic.

SNMPv2C includes a bulk retrieval mechanism and more detailed error message reporting to management stations. The bulk retrieval mechanism retrieves tables and large quantities of information, minimizing the number of round-trips required. The SNMPv2C improved error-handling includes expanded error codes that distinguish different kinds of error conditions; these conditions are reported through a single error code in SNMPv1. Error return codes now report the error type.

SNMPv3 provides for both security models and security levels. A security model is an authentication strategy that is set up for a user and the group in which the user resides. A security level is the permitted level of security within a security model. A combination of a security model and a security level will determine which security mechanism is employed when handling an SNMP packet.

You must configure the SNMP agent to use the version of SNMP supported by the management station. An agent can communicate with multiple managers; therefore, you can configure the software to support communications with one management station using the SNMPv1 protocol and another using the SNMPv2 protocol.

SNMP Manager Functions

The SNMP manager uses information in the MIB to perform the operations described in Table 17-1.

Table 17-1 SNMP Operations
Operation	Description
get-request	Retrieves a value from a specific variable.
get-next-request	Retrieves a value from a variable within a table.¹
get-bulk-request²	Retrieves large blocks of data that would otherwise require the transmission of many small blocks of data, such as multiple rows in a table.
get-response	Replies to a get-request, get-next-request, and set-request sent by an NMS.
set-request	Stores a value in a specific variable.
trap	An unsolicited message sent by an SNMP agent to an SNMP manager when some event has occurred.

¹With this operation, an SNMP manager does not need to know the exact variable name. A sequential search is performed to find the needed variable from within a table.

²The get-bulk command works only with SNMPv2.

SNMP Agent Functions

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Get a MIB variable—The SNMP agent begins this function in response to a request from the NMS. The agent retrieves the value of the requested MIB variable and responds to the NMS with that value.

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Set a MIB variable—The SNMP agent begins this function in response to a message from the NMS. The SNMP agent changes the value of the MIB variable to the value requested by the NMS.

The SNMP agent also sends unsolicited trap messages to notify an NMS that a significant event has occurred on the agent. Examples of trap conditions include, but are not limited to, when a port or module goes up or down, when spanning-tree topology changes occur, and when authentication failures occur.

SNMP Community Strings

SNMP community strings authenticate access to MIB objects and function as embedded passwords. In order for the NMS to access the bridge, the community string definitions on the NMS must match at least one of the three community string definitions on the bridge.

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Read-only—Gives read access to authorized management stations to all objects in the MIB except the community strings, but does not allow write access

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Read-write—Gives read and write access to authorized management stations to all objects in the MIB, but does not allow access to the community strings

Using SNMP to Access MIB Variables

An example of an NMS is the CiscoWorks network management software. CiscoWorks 2000 software uses the bridge MIB variables to set device variables and to poll devices on the network for specific information. The results of a poll can be displayed as a graph and analyzed to troubleshoot internetwork problems, increase network performance, verify the configuration of devices, monitor traffic loads, and more.

As shown in Figure 17-1, the SNMP agent gathers data from the MIB. The agent can send traps (notification of certain events) to the SNMP manager, which receives and processes the traps. Traps are messages alerting the SNMP manager to a condition on the network such as improper user authentication, restarts, link status (up or down), MAC address tracking, and so forth. The SNMP agent also responds to MIB-related queries sent by the SNMP manager in get-request, get-next-request, and set-request format.

Configuring SNMP

Default SNMP Configuration

Enabling the SNMP Agent

Table 17-2 Default SNMP Configuration
Feature	Default Setting
SNMP agent	Disabled
SNMP community strings	None configured
SNMP trap receiver	None configured
SNMP traps	None enabled

No specific IOS command exists to enable SNMP. The first snmp-server global configuration command that you enter enables SNMPv1 and SNMPv2.

Configuring Community Strings

You use the SNMP community string to define the relationship between the SNMP manager and the agent. The community string acts like a password to permit access to the agent on the bridge.

Optionally, you can specify one or more of these characteristics associated with the string:

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An access list of IP addresses of the SNMP managers that are permitted to use the community string to gain access to the agent

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A MIB view, which defines the subset of all MIB objects accessible to the given community

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Read and write or read-only permission for the MIB objects accessible to the community

Note In the current IOS MIB agent implementation, the default community string is for the Internet MIB object sub-tree. Because IEEE802dot11 is under another branch of the MIB object tree, you must enable either a separate community string and view on the IEEE802dot11 MIB or a common view and community string on the ISO object in the MIB object tree. ISO is the common parent node of IEEE (IEEE802dot11) and Internet. This MIB agent behavior is different from the MIB agent behavior on access points not running IOS software.

Beginning in privileged EXEC mode, follow these steps to configure a community string on the bridge:


	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	snmp-server community string [ access-list-number] [view mib-view] [ro \| rw]	Configure the community string. •For string, specify a string that acts like a password and permits access to the SNMP protocol. You can configure one or more community strings of any length. •(Optional) For access-list-number, enter an IP standard access list numbered from 1 to 99 and 1300 to 1999. •(Optional) For view mib-view, specify a MIB view to which this community has access, such as ieee802dot11. See the "Using the snmp-server view Command" section for instructions on using the snmp-server view command to access Standard IEEE 802.11 MIB objects through IEEE view. •(Optional) Specify either read-only (ro) if you want authorized management stations to retrieve MIB objects, or specify read/write (rw) if you want authorized management stations to retrieve and modify MIB objects. By default, the community string permits read-only access to all objects. Note To access the IEEE802dot11 MIB, you must enable either a separate community string and view on the IEEE802dot11 MIB or a common view and community string on the ISO object in the MIB object tree.
Step 3	access-list access-list-number {deny \| permit} source [source-wildcard]	(Optional) If you specified an IP standard access list number in Step 2, then create the list, repeating the command as many times as necessary. •For access-list-number, enter the access list number specified in Step 2. •The deny keyword denies access if the conditions are matched. The permit keyword permits access if the conditions are matched. •For source, enter the IP address of the SNMP managers that are permitted to use the community string to gain access to the agent. •(Optional) For source-wildcard, enter the wildcard bits in dotted decimal notation to be applied to the source. Place ones in the bit positions that you want to ignore. Recall that the access list is always terminated by an implicit deny statement for everything.
Step 4	end	Return to privileged EXEC mode.
Step 5	show running-config	Verify your entries.
Step 6	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To disable access for an SNMP community, set the community string for that community to the null string (do not enter a value for the community string). To remove a specific community string, use the no snmp-server community string global configuration command.

This example shows how to assign the strings open and ieee to SNMP, to allow read-write access for both, and to specify that open is the community string for queries on non-IEEE802dot11-MIB objects and ieee is the community string for queries on IEEE802dot11-mib objects:

Configuring Trap Managers and Enabling Traps

A trap manager is a management station that receives and processes traps. Traps are system alerts that the device generates when certain events occur. By default, no trap manager is defined, and no traps are issued.

Devices can have an unlimited number of trap managers. Community strings can be any length.

Table 17-3 describes the supported bridge traps (notification types). You can enable any or all of these traps and configure a trap manager to receive them.

Table 17-3 Notification Types
Notification Type	Description
authenticate-fail	Enable traps for authentication failures.
config	Enable traps for SNMP configuration changes.
deauthenticate	Enable traps for client device deauthentications.
disassociate	Enable traps for client device disassociations.
dot11-qos	Enable traps for QoS changes.
entity	Enable traps for SNMP entity changes.
envmon temperature	Enable traps for monitoring radio temperature. This trap is sent out when the bridge radio temperature approaches the limits of its operating range (55 C to -33 C; 131 F to -27.4 F).
linkDown	The interface keeps any DHCP-acquired IP address. Receipt of a valid linkDown trap starts a new link-down hold-down timer.
linkUp	When a linkUp trap event occurs, the DHCP client must either renew the current IP address or acquire a new IP address as quickly as possible.
snmp	Enable traps for SNMP events.
syslog	Enable syslog traps.
wlan-wep	Enable WEP traps.

Some notification types cannot be controlled with the snmp-server enable global configuration command, such as tty and udp-port. These notification types are always enabled. You can use the snmp-server host global configuration command to a specific host to receive the notification types listed in Table 17-3.

Beginning in privileged EXEC mode, follow these steps to configure the bridge to send traps to a host:


	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	snmp-server host host-addr {traps \| informs} {version {1 \| 2c \| 3}} community-string notification-type	Specify the recipient of the trap message. •For host-addr, specify the name or address of the host (the targeted recipient). •Specify traps (the default) to send SNMP traps to the host. Specify informs to send SNMP informs to the host. •Specify the SNMP version to support. Version 1, the default, is not available with informs. •For community-string, specify the string to send with the notification operation. Though you can set this string using the snmp-server host command, Cisco recommends that you define this string by using the snmp-server community command before using the snmp-server host command. •For notification-type, use the keywords listed in Table 17-3.
Step 3	snmp-server enable traps notification-types	Enable the bridge to send specific traps. For a list of traps, see Table 17-3. To enable multiple types of traps, you must issue a separate snmp-server enable traps command for each trap type.
Step 4	end	Return to privileged EXEC mode.
Step 5	show running-config	Verify your entries.
Step 6	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To remove the specified host from receiving traps, use the no snmp-server host host global configuration command. To disable a specific trap type, use the no snmp-server enable traps notification-types global configuration command.

Setting the Agent Contact and Location Information

Beginning in privileged EXEC mode, follow these steps to set the system contact and location of the SNMP agent so that these descriptions can be accessed through the configuration file:


	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	snmp-server contact text	Set the system contact string. For example: `snmp-server contact Dial System Operator at beeper 21555.`
Step 3	snmp-server location text	Set the system location string. For example: `snmp-server location Building 3/Room 222`
Step 4	end	Return to privileged EXEC mode.
Step 5	show running-config	Verify your entries.
Step 6	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Using the snmp-server view Command

In global configuration mode, use the snmp-server view command to access Standard IEEE 802.11 MIB objects through IEEE view and the dot11 read-write community string.

This example shows how to enable IEEE view and dot11 read-write community string:

SNMP Examples

This example shows how to enable SNMPv1 and SNMPv2C. The configuration permits any SNMP manager to access all objects with read-only permissions using the community string public. This configuration does not cause the bridge to send any traps.

This example shows how to permit any SNMP manager to access all objects with read-only permission using the community string public. The bridge also sends configuration traps to the hosts 192.180.1.111 and 192.180.1.33 using SNMPv1 and to the host 192.180.1.27 using SNMPv2C. The community string public is sent with the traps.

This example shows how to allow read-only access for all objects to members of access list 4 that use the comaccess community string. No other SNMP managers have access to any objects. SNMP Authentication Failure traps are sent by SNMPv2C to the host cisco.com using the community string public.

This example shows how to send Entity MIB traps to the host cisco.com. The community string is restricted. The first line enables the bridge to send Entity MIB traps in addition to any traps previously enabled. The second line specifies the destination of these traps and overwrites any previous snmp-server host commands for the host cisco.com.

This example shows how to enable the bridge to send all traps to the host myhost.cisco.com using the community string public:

Displaying SNMP Status

To display SNMP input and output statistics, including the number of illegal community string entries, errors, and requested variables, use the show snmp privileged EXEC command. For information about the fields in this display, refer to the Cisco IOS Configuration Fundamentals Command Reference for Release 12.2.

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