Understanding and Configuring SNMP on Cisco 3550

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

If the switch starts and the switch startup configuration has at least one snmp-server global configuration command, the SNMP agent is enabled.

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 the user and then adds it to the group associated with that user. Modifying the group’s notify view affects all users associated with that group. See the Cisco IOS Configuration Fundamentals Command Reference, Release 12.2 for information about when you should configure notify views.

•To configure a remote user, specify the IP address or port number for the remote SNMP agent of the device where the user resides.

•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 engineID 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.

Catalyst 3550 Multilayer Switch Software Configuration Guide