Catalyst 4500 Series Switch Software Configuration Guide, IOS XE 3.7.xE and IOS 15.2(3)Ex

SNMP Versions

The Catalyst 4500 series switch supports these SNMP versions:

• SNMPv1—The Simple Network Management Protocol, a Full Internet Standard, defined in RFC 1157.
• SNMPv2C replaces the Party-based Administrative and Security Framework of SNMPv2Classic with the community-string-based Administrative Framework of SNMPv2C while retaining the bulk retrieval and improved error handling of SNMPv2Classic. It has these features:

– SNMPv2—Version 2 of the Simple Network Management Protocol, a Draft Internet Standard, defined in RFCs 1902 through 1907.

– SNMPv2C—The community-string-based Administrative Framework for SNMPv2, an Experimental Internet Protocol defined in RFC 1901.

• SNMPv3—Version 3 of the SNMP is an interoperable standards-based protocol defined in RFCs 2273 to 2275. SNMPv3 provides secure access to devices by authenticating and encrypting packets over the network and includes these security features:

– Message integrity—Ensures that a packet was not tampered with in transit

– Authentication—Determines that the message is from a valid source

– Encryption—Mixes the contents of a package to prevent it from being read by an unauthorized source

Note To select encryption, enter the priv keyword. This keyword is available only when the crypto (encrypted) software image is installed.

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 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 in SNMPv2C report the error type.

SNMPv3 provides for both security models and security levels. A security model is an authentication strategy set up for a user and the group within which you reside. A security level is the permitted level of security within a security model. A combination of the security level and the security model determine which security mechanism is used when handling an SNMP packet. Available security models are SNMPv1, SNMPv2C, and SNMPv3.

The following table identifies the characteristics of the different combinations of security models and levels.

You must configure the SNMP agent to use the SNMP version supported by the management station. Because an agent can communicate with multiple managers, you can configure the software to support communications using SNMPv1, and SNMPv2C, and SNMPv3 protocols.

SNMP Manager Functions

The SNMP manager uses information in the MIB to perform the operations described in Table 65-1 .

SNMP Agent Functions

The SNMP agent responds to SNMP manager requests as follows:

• 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.
• 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 switch, the community string definitions on the NMS must match at least one of the three community string definitions on the switch.

A community string can have one of these attributes:

• Read-only (RO)—Gives read access to authorized management stations to all objects in the MIB except the community strings, but does not allow write access
• Read-write (RW)—Gives read and write access to authorized management stations to all objects in the MIB, but does not allow access to the community strings
• Read-write-all—Gives read and write access to authorized management stations to all objects in the MIB, including 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 switch 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 internetworking problems, increase network performance, verify the configuration of devices, monitor traffic loads, and more.

As shown in Figure 65-1, the SNMP agent gathers data from the MIB. The agent can send traps, or notification of certain events, to the SNMP manager, which receives and processes the traps. Traps alert 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.

Figure 65-1 SNMP Network

SNMP Notifications

SNMP allows the switch to send notifications to SNMP managers when particular events occur. SNMP notifications can be sent as traps or inform requests. In command syntax, unless there is an option in the command to select either traps or informs, the keyword traps refers to either traps or informs, or both. Use the snmp-server host command to specify whether to send SNMP notifications as traps or informs.

Note SNMPv1 does not support informs.

Traps are unreliable because the receiver does not send an acknowledgment when it receives a trap, and the sender cannot determine if the trap was received. When an SNMP manager receives an inform request, it acknowledges the message with an SNMP response protocol data unit (PDU). If the sender does not receive a response, the inform request can be sent again. Because they can be resent, informs are more likely than traps to reach their intended destination.

The characteristics that make informs more reliable than traps also consume more resources in the switch and in the network. Unlike a trap, which is discarded as soon as it is sent, an inform request is held in memory until a response is received or the request times out. Traps are sent only once, but an inform might be resent or retried several times. The retries increase traffic and contribute to a higher overhead on the network. Traps and informs require a trade-off between reliability and resources. If it is important that the SNMP manager receive every notification, use inform requests. If traffic on the network or memory in the switch is a concern and notification is not required, use traps.

Default SNMP Configuration

Table 65-2 shows the default SNMP configuration.

SNMP Configuration Guidelines

An SNMP group is a table that maps SNMP users to SNMP views. An SNMP user is a member of an SNMP group. An SNMP host is the recipient of an SNMP trap operation. An SNMP engine ID is a name for the local or remote SNMP engine.

When configuring SNMP, follow these guidelines:

• When configuring an SNMP group, do not specify a notify view. The snmp-server host global configuration command autogenerates a notify view for you and then adds it to the group associated with that user. Modifying the group's notify view affects all users associated with that group. For information about when you should configure notify views, see the Cisco IOS Configuration Fundamentals Command Reference, Release 12.2.
• To configure a remote user, specify the IP address or port number for the remote SNMP agent of the device where you reside.
• Before you configure remote users for a particular agent, configure the SNMP engine ID, using the snmp-server engineID global configuration with the remote option. The remote agent's SNMP engine ID and user password are used to compute the authentication and privacy digests. If you do not configure the remote engine ID first, the configuration command fails.
• When configuring SNMP informs, you need to configure the SNMP engine ID for the remote agent in the SNMP database before you can send proxy requests or informs to it.
• If a local user is not associated with a remote host, the switch does not send informs for the auth (authNoPriv) and the priv (authPriv) authentication levels.
• Changing the value of the SNMP engine ID has important side effects. A user's password (entered on the command line) is converted to an MD5 or SHA security digest based on the password and the local engine ID. The command-line password is then destroyed, as required by RFC 2274. Because of this deletion, if the value of the engine ID changes, the security digests of SNMPv3 users become invalid, and you need to reconfigure SNMP users by using the snmp-server user username global configuration command. Similar restrictions require the reconfiguration of community strings when the engine ID changes.

Disabling the SNMP Agent

To disable the SNMP agent, perform this task:

The no snmp-server global configuration command disables all running versions (Version 1, Version 2C, and Version 3) on the device. No specific Cisco IOS command exists to enable SNMP. The first snmp-server global configuration command that you enter enables all versions of SNMP.

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

• An access list of IP addresses of the SNMP managers that are permitted to use the community string to gain access to the agent
• A MIB view, which defines the subset of all MIB objects accessible to the given community
• Read and write or read-only permission for the MIB objects accessible to the community

To configure a community string on the switch, perform this task:

Note 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).

Note You cannot use the snmp-server enable informs command. To enable the sending of SNMP inform notifications, use the snmp-server enable traps command combined with the snmp-server host host-addr informs command.

This example shows how to assign the string comaccess to SNMP, to allow read-only access, and to specify that IP access list 4 can use the community string to gain access to the switch SNMP agent:

Configuring SNMP Groups and Users

You can specify an identification name (engine ID) for the local or remote SNMP server engine on the switch. You can configure an SNMP server group that maps SNMP users to SNMP views, and you can add new users to the SNMP group.

To configure SNMP on the switch, perform this task:

Configuring SNMP Notifications

A trap manager is a management station that receives and processes traps. Traps are system alerts that the switch generates when certain events occur. By default, no trap manager is defined, and no traps are sent. Switches running Cisco IOS Release 12.2(31)SG and later releases can have an unlimited number of trap managers.

Note Many commands use the word traps in the command syntax. Unless there is an option in the command to select either traps or informs, the keyword traps refers to either traps, informs, or both. Use the snmp-server host command to specify whether to send SNMP notifications as traps or informs.

Table 65-3 describes the supported switch traps (notification types). You can enable any or all of these traps and configure a trap manager to receive them.

You can use the snmp-server host global configuration command to a specific host to receive the notification types listed in Table 65-3 .

To configure the switch to send traps or informs to a host, perform this task:

The snmp-server host command specifies which hosts receive the notifications. The
snmp-server enable trap command globally enables the mechanism for the specified notification (for traps and informs). To enable a host to receive an inform, you must configure an
snmp-server host informs command for the host and globally enable informs by using the
snmp-server enable traps command.

To remove the specified host from receiving traps, use the no snmp-server host host global configuration command. The no snmp-server host command with no keywords disables traps, but not informs, to the host. To disable informs, use the no snmp-server host informs 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

To set the system contact and location of the SNMP agent so that these descriptions can be accessed using the configuration file, perform this task:

Limiting TFTP Servers Used Through SNMP

To limit the TFTP servers used for saving and loading configuration files through SNMP to the servers specified in an access list, perform this task:

SNMP Examples

This example shows how to enable all versions of SNMP. 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 switch 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 switch also sends VTP 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 switch 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 switch to send all traps to the host myhost.cisco.com using the community string public :

This example shows how to associate a user with a remote host and to send auth (authNoPriv) authentication-level informs when you enter global configuration mode: