SNMP is an application-layer protocol that provides a message format for communication between SNMP managers and agents. SNMP provides a standardized framework and a common language used for monitoring and managing devices in a network.

The SNMP framework is made up of three parts:

• SNMP Manager
  
• SNMP Agent
  
• MIB
  

SNMP applications perform the following operations to retrieve data, modify SNMP object variables, and send notifications:

• SNMP Get
  
• SNMP Set
  
• SNMP Notifications
  

MIB modules typically are defined in RFC documents submitted to the IETF, an international standards body. RFCs are written by individuals or groups for consideration by the Internet Society and the Internet community as a whole, usually with the intention of establishing a recommended Internet standard. Before being given RFC status, recommendations are published as Internet Draft (I-D) documents. RFCs that have become recommended standards are also labeled as standards documents (STDs). You can learn about the standards process and the activities of the IETF at the Internet Society website at http://www.isoc.org. You can read the full text of all RFCs, I-Ds, and STDs referenced in Cisco documentation at the IETF website at http://www.ietf.org.

The Cisco implementation of SNMP uses the definitions of MIB II variables described in RFC 1213 and definitions of SNMP traps described in RFC 1215.

Cisco provides its own private MIB extensions with every system. Cisco enterprise MIBs comply with the guidelines described in the relevant RFCs unless otherwise noted in the documentation. You can find the MIB module definition files and the list of MIBs supported on each Cisco platform on the Cisco MIB website on Cisco.com.

The Cisco IOS software supports the following versions of SNMP:

• SNMPv1--Simple Network Management Protocol: a full Internet standard, defined in RFC 1157. (RFC 1157 replaces the earlier versions that were published as RFC 1067 and RFC 1098.) Security is based on community strings.
• SNMPv2c--The community string-based Administrative Framework for SNMPv2. SNMPv2c (the "c" is for "community") is an experimental Internet protocol defined in RFC 1901, RFC 1905, and RFC 1906. SNMPv2c is an update of the protocol operations and data types of SNMPv2p (SNMPv2 Classic) and uses the community-based security model of SNMPv1.
• SNMPv3--Version 3 of SNMP. SNMPv3 is an interoperable standards-based protocol defined in RFCs 3413 to 3415. SNMPv3 provides secure access to devices by authenticating and encrypting packets over the network.

The security features provided in SNMPv3 are as follows:

• • Message integrity--Ensuring that a packet has not been tampered with in transit.
  • Authentication--Determining that the message is from a valid source.
  • Encryption--Scrambling the contents of a packet to prevent it from being learned by an unauthorized source.

Both SNMPv1 and SNMPv2c use a community-based form of security. The community of SNMP managers able to access the agent MIB is defined by an IP address access control list (ACL) and password.

SNMPv2c support includes a bulk retrieval mechanism and detailed error message reporting to management stations. The bulk retrieval mechanism supports the retrieval of tables and large quantities of information, minimizing the number of round trips required. The SNMPv2c improved error handling support includes expanded error codes that distinguish different types of errors; these conditions are reported through a single error code in SNMPv1. The following three types of exceptions are also reported: no such object, no such instance, and end of MIB view.

SNMPv3 is a security model in which an authentication strategy 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 determines which security mechanism is employed when handling an SNMP packet.

Three security models are available: SNMPv1, SNMPv2c, and SNMPv3. The table below lists the combinations of security models and levels and their meanings.

Note	SNMPv2p (SNMPv2 Classic) is not supported in Cisco IOS Release 11.2 and later releases. SNMPv2c replaces the Party-based Administrative and Security Framework of SNMPv2p with a Community-based Administrative Framework. SNMPv2c retained the bulk retrieval and error handling capabilities of SNMPv2p.

You must configure an SNMP agent to use the version of SNMP supported by the management station. An agent can communicate with multiple managers. You can configure the Cisco IOS software to support communications with one management station using the SNMPv1 protocol, one using the SNMPv2c protocol, and another using SMNPv3.

SNMPv3 supports RFCs 1901 to 1908, 2104, 2206, 2213, 2214, and 2271 to 2275. For additional information about SNMPv3, see RFC 2570, Introduction to Version 3 of the Internet-standard Network Management Framework (this document is not a standard).

SNMPv3 provides different levels of security. If an authentication or an authorization request fails, a descriptive error message is returned to indicate what went wrong. These error messages are RFC 3414 compliant.

You can use the snmp-server usm cisco command to disable the descriptive messages to prevent malicious users from misusing the information returned in the error messages. The table below lists the error messages returned when the snmp-server usm cisco command is used and compares these messages with the corresponding RFC-compliant error messages. The Cisco-specific error messages are a deviation from RFC 3414.

Note	For a configured SNMP user, if the SNMP group is not configured or the group security level is not the same as the user security level, the error returned is AUTHORIZATION_ERROR. The Cisco-specific error message for this scenario is unknownUserName.

The Interface Index Display for SNMP feature introduces new commands and command modifications that allow advanced users of SNMP to view information about the interface registrations directly on the managed agent. You can display MIB information from the agent without using an external NMS.

Note	For the purposes of this document, the agent is a routing device running Cisco IOS software.

This feature addresses three objects in the Interfaces MIB: ifIndex, ifAlias, and ifName. For a complete definition of these objects, see the IF-MIB.my file available from the Cisco SNMPv2 MIB website at ftp://ftp.cisco.com/pub/mibs/v2/.

• Interface Index
  
• Interface Alias
  
• Interface Name
  

The SNMP Support for VPNs feature allows SNMP traps and informs to be sent and received using VPN routing and forwarding (VRF) tables. In particular, this feature adds support to the Cisco IOS software for sending and receiving SNMP traps and informs specific to individual VPNs.

A VPN is a network that provides high connectivity transfers on a shared system with the same usage guidelines as a private network. A VPN can be built on the Internet over IP, Frame Relay, or ATM networks.

A VRF stores per-VPN routing data. It defines the VPN membership of a customer site attached to the network access server (NAS). A VRF consists of an IP routing table, a derived Cisco Express Forwarding table, and guidelines and routing protocol parameters that control the information that is included in the routing table.

The SNMP Support for VPNs feature provides configuration commands that allow users to associate SNMP agents and managers with specific VRFs. The specified VRF is used for sending SNMP traps and informs and responses between agents and managers. If a VRF is not specified, the default routing table for the VPN is used.

Support for VPNs allows you to configure an SNMP agent to accept only SNMP requests from a certain set of VPNs. With this configuration, service providers can provide network management services to their customers, so customers can manage all user VPN devices.

One of the identifiers most commonly used in SNMP-based network management applications is the interface index (IfIndex) value . IfIndex is a unique identifying number associated with a physical or logical interface; as far as most software is concerned, the ifIndex is the name of the interface.

Although there is no requirement in the relevant RFCs that the correspondence between particular ifIndex values and their interfaces be maintained across reboots, applications such as device inventory, billing, and fault detection increasingly depend on the maintenance of this correspondence.

This feature adds support for an ifIndex value that can persist across reboots, allowing users to avoid the workarounds previously required for consistent interface identification.

It is currently possible to poll the router at regular intervals to correlate the interfaces to the ifIndex, but it is not practical to poll this interface constantly. If this data is not correlated constantly, however, the data may be made invalid because of a reboot or the insertion of a new card into the router in between polls. Therefore, ifIndex persistence is the only way to guarantee data integrity.

IfIndex persistence means that the mapping between the ifDescr object values and the ifIndex object values (generated from the IF-MIB) will be retained across reboots.

• Benefits of Interface Index Persistence
  

The MIB Persistence features allow the SNMP data of a MIB to be persistent across reloads; that is, MIB information retains the same set object values each time a networking device reboots. MIB Persistence is enabled by issuing the snmp mib persist command, and the MIB data of all MIBs that have had persistence enabled using this command is then written to NVRAM by issuing the write mib-data command. All modified MIB data must be written to NVRAM using the write mib-data command.

Both Event and Expression MIBs allow you to configure a value for an object and to set up object definitions. Both also allow rows of data to be modified while the row is in an active state.

Scalar objects are stored every time they are changed, and table entries are stored only if the row is in an active state. The Event MIB has two scalar objects and nine tables to be persisted into NVRAM. The tables are as follows:

• mteEventNotificationTable
• mteEventSetTable
• mteEventTable
• mteObjectsTable
• mteTriggerBooleanTable
• mteTriggerDeltaTable
• mteTriggerExistenceTable
• mteTriggerTable
• mteTriggerThresholdTable

The Expression MIB has two scalar objects and three tables to be stored in NVRAM. The scalar objects are expResourceDeltaMinimum and expResourceDeltaWildcardInstanceMaximum. The tables are as follows:

• expExpressionTable
• expNameTable
• expObjectTable

Writing MIB data to NVRAM may take several seconds. The length of time depends on the amount of MIB data.

Event MIB Persistence and Expression MIB Persistence both allow MIB objects to be saved from reboot to reboot, allowing long-term monitoring of specific devices and interfaces and configurations of object values that are preserved across reboots.

The Circuit Interface MIB (CISCO-CIRCUIT-INTERFACE-MIB) provides a MIB object (cciDescr) that can be used to identify individual circuit-based interfaces for SNMP monitoring. The Circuit Interface Identification Persistence for SNMP feature maintains this user-defined name of the circuit across reboots, allowing the consistent identification of circuit interfaces. Circuit Interface Identification Persistence is enabled using the snmp mib persist circuit global configuration command.

Cisco IOS Release 12.2(2)T introduces the Circuit Interface Identification Persistence for SNMP feature. The Circuit Interface MIB (CISCO-CIRCUIT-INTERFACE-MIB) provides a MIB object (cciDescr) that can be used to identify individual circuit-based interfaces for SNMP monitoring. The Cisco Circuit Interface MIB was introduced in Cisco IOS Release 12.1(3)T.

The Circuit Interface Identification Persistence for SNMP feature maintains the user-defined name of the circuit (defined in the cciDescr object) across reboots, allowing for the consistent identification of circuits.

The Circuit Interface Identification Persistence for SNMP feature is a supplement to the Interface Index Persistence feature introduced in Cisco IOS Release 12.1(3)T and Cisco IOS Release 12.0(11)S. Circuit Interface Identification Persistence is enabled using the snmp mib persist circuit global configuration command. Use this command if you need to consistently identify circuits using SNMP across reboots. This command is disabled by default because this feature uses NVRAM.

In addition, the show snmp mib ifmib ifindex EXEC mode command allows you to display the Interfaces MIB ifIndex values directly on your system without an NMS; the show snmp mib EXEC mode command allows you to display a list of MIB module identifiers registered directly on your system with an NMS. The snmp ifmib ifalias long command allows you to specify a description for interfaces or subinterfaces of up to 256 characters in length. Prior to the introduction of this command, ifAlias descriptions for SNMP management were limited to 64 characters.

The Event MIB provides the ability to monitor MIB objects on a local or remote system using SNMP and initiate simple actions whenever a trigger condition is met; for example, an SNMP trap can be generated when an object is modified. When the notifications are triggered through events, the NMS does not need to constantly poll managed devices to track changes.

By allowing the SNMP notifications to take place only when a specified condition is met, the Event MIB reduces the load on affected devices and improves the scalability of network management solutions.

The Event MIB operates based on event, object lists configured for the event, event action, trigger, and trigger test.

• Events
  
• Object List
  
• Trigger
  
• Trigger Test
  

The Expression MIB allows you to create expressions based on a combination of objects. The expressions are evaluated according to the sampling method. The Expression MIB supports the following types of object sampling:

• Absolute
• Delta
• Changed

If there are no delta or change values in an expression, the expression is evaluated when a requester attempts to read the value of the expression. In this case, all requesters get a newly calculated value.

For expressions with delta or change values, an evaluation is performed for every sampling. In this case, requesters get the value as of the last sample period.

• Absolute Sampling
  
• Delta Sampling
  
• Changed Sampling
  

Systems that support SNMP often need a mechanism for recording notification information. This mechanism protects against notifications being lost because they exceeded retransmission limits. The Notification Log MIB provides a common infrastructure for other MIBs in the form of a local logging function. The SNMP Notification Logging feature adds Cisco IOS CLI commands to change the size of the notification log, to set the global ageout value for the log, and to display logging summaries at the command line. The Notification Log MIB improves notification tracking and provides a central location for tracking all MIBs.

Note	The Notification Log MIB supports notification logging on the default log only.

1.    enable

2.    configure terminal

3.    snmp-server contact text

4.    snmp-server location text

5.    snmp-server chassis-id number

6.    exit

7.    show snmp contact

8.    show snmp location

9.    show snmp chassis

When you configure SNMP versions 1 and 2, you can optionally create or modify views for community strings to limit which MIB objects an SNMP manager can access.

Perform the following tasks when configuring SNMP version 1 or version 2.

• Prerequisites
  
• Creating or Modifying an SNMP View Record
  
• Creating or Modifying Access Control for an SNMP Community
  
• Configuring a Recipient of an SNMP Trap Operation
  

When you configure SNMPv3 and you want to use the SNMPv3 security mechanism for handling SNMP packets, you must establish SNMP groups and users with passwords.

Perform the following tasks to configure SNMPv3.

• Specifying SNMP-Server Group Names
  
• Configuring SNMP Server Users
  

1.    enable

2.    configure terminal

3.    snmp-server manager

4.    snmp-server manager session-timeout seconds

5.    exit

6.    show snmp

7.    show snmp sessions brief

8.    show snmp pending

Router# show snmp

Chassis: 01506199
37 SNMP packets input
    0 Bad SNMP version errors
    4 Unknown community name
    0 Illegal operation for community name supplied
    0 Encoding errors
    24 Number of requested variables
    0 Number of altered variables
    0 Get-request PDUs
    28 Get-next PDUs
    0 Set-request PDUs
78 SNMP packets output
    0 Too big errors (Maximum packet size 1500)
    0 No such name errors
    0 Bad values errors
    0 General errors
    24 Response PDUs
    13 Trap PDUs
SNMP logging: enabled
    Logging to 172.17.58.33.162, 0/10, 13 sent, 0 dropped.
SNMP Manager-role output packets
    4 Get-request PDUs
    4 Get-next PDUs
    6 Get-bulk PDUs
    4 Set-request PDUs
    23 Inform-request PDUs
    30 Timeouts
    0 Drops
SNMP Manager-role input packets
    0 Inform response PDUs
    2 Trap PDUs
    7 Response PDUs
    1 Responses with errors
SNMP informs: enabled
    Informs in flight 0/25 (current/max)
    Logging to 172.17.217.141.162
        4 sent, 0 in-flight, 1 retries, 0 failed, 0 dropped
    Logging to 172.17.58.33.162
        0 sent, 0 in-flight, 0 retries, 0 failed, 0 dropped

Router# show snmp sessions

Destination: 172.17.58.33.162, V2C community: public
  Round-trip-times: 0/0/0 (min/max/last)
  packets output
    0 Gets, 0 GetNexts, 0 GetBulks, 0 Sets, 4 Informs
    0 Timeouts, 0 Drops
  packets input
    0 Traps, 0 Informs, 0 Responses (0 errors)
Destination: 172.17.217.141.162, V2C community: public, Expires in 575 secs
  Round-trip-times: 1/1/1 (min/max/last)
  packets output
    0 Gets, 0 GetNexts, 0 GetBulks, 0 Sets, 4 Informs
    0 Timeouts, 0 Drops
 packets input
    0 Traps, 0 Informs, 4 Responses (0 errors)

Router# show snmp pending

req id: 47, dest: 172.17.58.33.161, V2C community: public, Expires in 5 secs
req id: 49, dest: 172.17.58.33.161, V2C community: public, Expires in 6 secs
req id: 51, dest: 172.17.58.33.161, V2C community: public, Expires in 6 secs
req id: 53, dest: 172.17.58.33.161, V2C community: public, Expires in 8 secs

1.    enable

2.    configure terminal

3.    snmp-server system-shutdown

4.    exit

1.    enable

2.    configure terminal

3.    snmp-server packetsize byte-count

4.    exit

1.    enable

2.    configure terminal

3.    snmp-server tftp-server-list number

• Troubleshooting Tips
  

1.    enable

2.    configure terminal

3.    no snmp-server

To configure a router to send SNMP traps or informs, perform the tasks described in the following sections:

Note

Many snmp-server commands use the word traps in their command syntax. Unless there is an option within the command to specify either traps or informs, the keyword traps should be taken to mean traps, informs, or both. Use the snmp-server host command to specify whether you want SNMP notifications to be sent as traps or informs. To use informs, the SNMP manager (also known as the SNMP proxy manager) must be available and enabled on a device. Earlier, the SNMP manager was available only with Cisco IOS PLUS images. However, the SNMP manager is now available with all Cisco IOS releases that support SNMP. Use Cisco Feature Navigator for information about SNMP manager support for Cisco IOS releases. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn.

• Configuring the Router to Send SNMP Notifications
  
• Changing Notification Operation Values
  
• Controlling Individual RFC 1157 SNMP Traps
  
• Configuring SNMP Notification Log Options
  

Before You Begin

SNMP must be enabled on your system.

The Interface Index Display and Interface Alias Long Name Support feature is not supported on all Cisco platforms. Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support.

Perform this task to configure the IF-MIB to retain ifAlias values of longer than 64 characters and to configure the ifAlias values for an interface.

Note

To verify if the ifAlias description is longer than 64 characters, perform an SNMP MIB walk for the ifMIB ifAlias variable from an NMS and verify that the entire description is displayed in the values for ifXEntry.18. The description for interfaces also appears in the output from the more system:running config privileged EXEC mode command.

1.    enable

2.    configure terminal

3.    snmp ifmib ifalias long

4.    interface type number

5.    description text-string

6.    exit

7.    show snmp mib

8.    show snmp mib ifmib ifindex [type number] [detail] [free-list]

Router# show snmp mib
system.1
system.2
sysUpTime
system.4
system.5
system.6
system.7
system.8
sysOREntry.2
sysOREntry.3
sysOREntry.4
interfaces.1
ifEntry.1
ifEntry.2
ifEntry.3
ifEntry.4
ifEntry.5
ifEntry.6
ifEntry.7
ifEntry.8
ifEntry.9
ifEntry.10
ifEntry.11
 --More-- 
captureBufferEntry.2
captureBufferEntry.3
captureBufferEntry.4
captureBufferEntry.5
captureBufferEntry.6
captureBufferEntry.7
capture.3.1.1
eventEntry.1
eventEntry.2
eventEntry.3
eventEntry.4
eventEntry.5
eventEntry.6
eventEntry.7
logEntry.1
logEntry.2
logEntry.3
logEntry.4
rmon.10.1.1.2
rmon.10.1.1.3
rmon.10.1.1.4
rmon.10.1.1.5
rmon.10.1.1.6
rmon.10.1.1.7
rmon.10.2.1.2
rmon.10.2.1.3
rmon.10.3.1.2

Router# show snmp mib ifmib ifindex Ethernet 2/0
Ethernet2/0: Ifindex = 2

Router# show snmp mib ifmib ifindex
ATM1/0: Ifindex = 1
ATM1/0-aal5 layer: Ifindex = 12
ATM1/0-atm layer: Ifindex = 10
ATM1/0.0-aal5 layer: Ifindex = 13
ATM1/0.0-atm subif: Ifindex = 11
ATM1/0.9-aal5 layer: Ifindex = 32
ATM1/0.9-atm subif: Ifindex = 31
ATM1/0.99-aal5 layer: Ifindex = 36
ATM1/0.99-atm subif: Ifindex = 35
Ethernet2/0: Ifindex = 2
Ethernet2/1: Ifindex = 3
Ethernet2/2: Ifindex = 4
Ethernet2/3: Ifindex = 5
Null0: Ifindex = 14
Serial3/0: Ifindex = 6
Serial3/1: Ifindex = 7
Serial3/2: Ifindex = 8
Serial3/3: Ifindex = 9

• Troubleshooting Tips
  

This section describes how to configure SNMP support for VPNs. The SNMP Support for VPNs feature provides configuration commands that allow users to associate SNMP agents and managers with specific VRFs. The specified VRF is used to send SNMP traps and informs and responses between agents and managers. If a VRF is not specified, the default routing table for the VPN is used.

Support for VPNs allows users to configure an SNMP agent to only accept SNMP requests from a certain set of VPNs. With this configuration, providers can provide network management services to their customers who then can manage all user-VPN devices.

Note

This feature is not supported on all Cisco platforms. Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support. Not all MIBs are VPN-aware. To list the VPN-aware MIBs, use the show snmp mib context command. For more information about VPN-aware MIBs, see the SNMP Support over VPNs--Context-based Access Control configuration module.

Perform this task to configure SNMP support for a specific VPN.

1.    enable

2.    configure terminal

3.    snmp-server host host-address [vrf vrf-name] [traps | informs] [version {1| 2c| 3 [auth | noauth |priv]}] community-string [udp-port port] [notification-type]

4.    snmp-server engineID remote ip-address [udp-port udp-port-number] [vrf vrf-name] engineid-string

5.    exit

6.    show snmp host

The following sections contain the tasks to configure Interface Index Persistence:

• Enabling and Disabling IfIndex Persistence Globally
  
• Enabling and Disabling IfIndex Persistence on Specific Interfaces
  

Note	Beginning with Cisco IOS Release 12.4(20)T, MIB persistence is automatic; manual configuration is not required.

The MIB Persistence features allow the SNMP data of a MIB to be persistent across reloads; that is, MIB information retains the same set of object values each time a networking device reboots. The following sections contain tasks for using Distributed Management Event and Expression MIB persistence.

• Prerequisites
  
• Restrictions
  
• Enabling and Disabling Event MIB Persistence
  
• Enabling and Disabling Expression MIB Persistence
  

The Event MIB can be configured using SNMP directly. In this procedure, the Event MIB is configured to monitor the delta values of ifInOctets for all interfaces once per minute. If any of the samples exceed the specified threshold, a trap notification will be sent.

There are no Cisco IOS software configuration tasks associated with the Event MIB. All configuration of Event MIB functionality must be performed though applications using SNMP. This section provides a sample configuration session using a network management application on an external device. See the Additional References section for information about configuring SNMP on your Cisco routing device.

All configuration of Event MIB functionality must be performed though applications using SNMP. The following section provides a step-by-step Event MIB configuration using SNMP research tools available for Sun workstations. The setany commands given below are executed using the SNMP application. Note that these commands are not Cisco IOS CLI commands. It is assumed that SNMP has been configured on your routing device.

In this configuration, the objective is to monitor ifInOctets for all interfaces. The Event MIB is configured to monitor the delta values of ifInOctets for all interfaces once per minute. If any of the samples exceed the specified threshold of 30, a Trap notification will be sent.

There are five parts to the following example:

• Setting the Trigger in the Trigger Table
  
• Creating an Event in the Event Table
  
• Setting the Trigger Threshold in the Trigger Table
  
• Activating the Trigger
  
• Monitoring and Maintaining Event MIB
  

The Event MIB can be configured using SNMP directly. In this procedure, the Event MIB is configured to monitor delta values of ifInOctets for all interfaces once per minute. If any of the samples exceed the specified threshold, a trap notification will be sent.

However, in Cisco IOS Release 12.4(20)T, the Event MIB feature is enhanced to add CLIs to configure the events, event action, and trigger.

This section contains the following tasks to configure the Event MIB:

• Configuring Scalar Variables
  
• Configuring Event MIB Object List
  
• Configuring Event
  
• Configuring Event Action
  
• Configuring Event Trigger
  
• Configuring Existence Trigger Test
  
• Configuring Boolean Trigger Test
  
• Configuring Threshold Trigger Test