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Table Of Contents
Related Features and Technologies
Supported Standards, MIBs, and RFCs, and MIBs
Enabling and Disabling IfIndex Persistence Globally
Enabling and Disabling IfIndex Persistence on Specific Interfaces
Enabling IfIndex Persistence on All Interfaces Example
Enabling IfIndex Persistence on a Specific Interface Example
Disabling IfIndex Persistence on a Specific Interface Example
Clearing IfIndex Persistence Configuration from a Specific Interface Example
Interface Index Persistence
This feature module describes the Interface Index Persistence feature introduced in Cisco IOS Release 12.1(5)T. This enhancement allows interfaces to be identified with unique values which will remain constant even when a device is rebooted. These interface identification values are used for network monitoring and management using Simple Network Management Protocol (SNMP). This document includes the following sections:
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Supported Standards, MIBs, and RFCs, and MIBs
Feature Overview
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.
Cisco IOS Release 12.1(5)T 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
Association of Interfaces with Traffic Targets for Network Management
The Interface Index Persistence feature allows for greater accuracy when collecting and processing network management data by uniquely identifying input and output interfaces for traffic flows and SNMP statistics. Relating each interface to a known entity (such as an ISP customer) allows network management data to be more effectively utilized.
Accuracy for Mediation, Fault Detection, and Billing
Network data is increasingly being used worldwide for usage-based billing, network planning, policy enforcement, and trend analysis. The ifIndex information is used to identify input and output interfaces for traffic flows and SNMP statistics. Inability to reliably relate each interface to a known entity, such as a customer, invalidates the data.
Restrictions
The interface-specific ifIndex persistence command ([no] snmp ifindex persistence) cannot be used on subinterfaces. A command applied to an interface is automatically applied to all the subinterfaces associated with that interface.
Testing indicates that approximately 25 bytes of NVRAM storage are used by this feature per interface. There may be some boot delay exhibited on platforms with lower CPU speeds.
Related Features and Technologies
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Related Documents
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RFCs are available from a variety of internet sources. The primary source is the IETF's web site at http://www.ietf.org
Supported Platforms
This feature is supported on images for the following platforms in Cisco IOS release 12.1(5)T and later:
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The Interface Index Persistence feature is also supported on the following platfroms in Cisco IOS release 12.0(11)S and later:
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For more information about the Cisco IOS software release process, and the latest information about feature support by platform, see Cisco.com.
Supported Standards, MIBs, and RFCs, and MIBs
The Interface Index Persistence feature supports the following standards, MIBs, and RFCs:
Standards
None (ifIndex persistence is not required by RFC2233).
MIBs
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Note that this feature does not change any existing MIBs or add any new MIBs.
To obtain lists of MIBs supported by platform and Cisco IOS release, and to download MIB modules, go to the Cisco MIB website on Cisco.com at http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml.
RFCs
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Prerequisites
The configuration tasks described in the next section assume that you have configured SNMP on your routing device and are using SNMP to monitor network activity using the Cisco IOS command line interface and/or a network management system (NMS) application.
Configuration Tasks
See the following sections for configuration tasks for the Interface Index Persistence feature. Each task in the list is identified as optional or required.
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Enabling and Disabling IfIndex Persistence Globally
IfIndex persistence is disabled by default. To globally enable ifIndex values that are maintained across reboots, use the following command in global configuration mode:
Router(config)# snmp-server ifindex persist
Globally enables ifIndex values that will remain constant across reboots.
To globally disable ifIndex persistence after enabling it, use the following command in global configuration mode:
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Enabling and Disabling IfIndex Persistence on Specific Interfaces
To enable ifIndex persistence only on a specific interface, use the following commands, starting in global configuration mode:
To disable ifIndex persistence only on a specific interface, use the following commands, starting in global configuration mode:
To clear the interface-specific ifIndex persistence setting and configure the interface to use the global configuration setting, use the following commands, starting in global configuration mode:
When you clear the interface-specific setting, only the global ifIndex persistence setting will apply to the interface. Regardless of whether the specific interface has ifIndex persistence enabled or disabled, the ifIndex persistence setting will default to the global setting after you issue the snmp ifindex clear command.
For example, assumethat you enabled ifIndex persistence on interface ethernet 0/1, and then globally enabled ifIndex persistence. Using the snmp ifindex clear command in interface configuration mode for ethernet 0/1 would leave that interface with ifIndex enabled, because the global setting is to have ifIndex persistence enabled.
Likewise, if you disabled ifIndex persistence for ethernet 0/1, globally enabled ifIndex persistence, and then issued the snmp ifindex clear command on that interface, ifIndex would be enabled (according to the global setting) on that interface.
Tips
Verifying IfIndex Persistence
Use the more system:running-config command to verify that ifIndex commands have been configured.
Configuration Examples
This section provides the following configuration examples:
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Enabling IfIndex Persistence on All Interfaces Example
In the following example, ifIndex persistence is enabled for all interfaces:
router(config)# snmp-server ifindex persistEnabling IfIndex Persistence on a Specific Interface Example
In the following example, ifIndex persistence is enabled for interface ethernet 0/1 only:
router(config)# interface ethernet 0/1router(config-if)# snmp ifindex persistrouter(config-if)# exitDisabling IfIndex Persistence on a Specific Interface Example
In the following example, ifIndex persistence is disabled for interface ethernet 0/1 only:
router(config)# interface ethernet 0/1router(config-if)# no snmp ifindex persistrouter(config-if)# exitClearing IfIndex Persistence Configuration from a Specific Interface Example
In the following example, any previous setting for ifIndex persistence on interface ethernet 0/1 is removed from the configuration. If ifIndex persistence is globally enabled, ifIndex persistence will be enabled for ethernet 0/1. If ifIndex persistence is globally disabled, ifIndex persistence will be disabled for ethernet 0/1.
router(config)# interface ethernet 0/1router(config-if)# snmp ifindex clearrouter(config-if)# exitCommand Reference
This section documents new commands. All other commands used with this feature are documented in the Cisco IOS Release 12.1 command reference publications.
snmp ifindex clear
To clear any previously configured snmp ifIndex commands issued in interface configuration mode for a specific interface, use the snmp ifindex clear command in interface configuration mode.
snmp ifindex clear
Syntax Description
This command has no arguments or keywords.
Defaults
None
Command Modes
Interface configuration mode
Command History
12.0(11)S
This command was introduced.
12.1(5)T
This command was integrated into the 12.1 T release train.
Usage Guidelines
Interface Index Persistence means that ifIndex values in the IF-MIB persist across reboots, allowing for consistent indentification of specific interfaces using SNMP.
Use the snmp ifindex clear command on a specific interface when you want that interface to use the global configuration setting for ifIndex persistence. This command clears any ifIndex configuration commands previously entered for that specific interface.
Examples
In the following example, ifIndex persistence is enabled for all interfaces:
router(config)# snmp-server ifindex persistIfIndex persistence is then disabled for ethernet 0/1 only:
router(config)# interface ethernet 0/1router(config-if)# no snmp ifindex persistrouter(config-if)# exitLater, the ifIndex configuration command is cleared from the configuration for ethernet 0/1:
router(config)# interface ethernet 0/1router(config-if)# snmp ifindex clearrouter(config-if)# exitThis leaves ifIndex persistence enabled for all interfaces, as specified by the snmp-server ifindex persist global configuration command.
Related Commands
snmp ifindex persist
To enable ifIndex values in the Interfaces MIB (IF-MIB) that persist across reboots (ifIndex persistence) only on a specific interface, use the snmp ifindex persist command in interface configuration mode. To disable ifIndex persistence only a specific interface, use the no form of this command.
snmp ifindex persist
no snmp ifindex persist
Syntax Description
This command has no arguments or keywords.
Defaults
This command is disabled by default.
Command Modes
Interface configuration mode
Command History
12.0(11)S
This command was introduced.
12.1(5)T
This command was integrated into the 12.1 T release train.
Usage Guidelines
Interface Index Persistence means that ifIndex values in the IF-MIB persist across reboots, allowing for consistent indentification of specific interfaces using SNMP.
The snmp ifindex persistence interface configuration command enables and disables ifIndex persistence for individual entries (corresponding to individual interfaces) in the ifIndex table of the IF-MIB.
The snmp-server ifindex persistence global configuration command enables and disables ifIndex persistence for all interfaces on the routing device (this applies only to interfaces that have ifDescr and ifIndex entries in the ifIndex table of the IF-MIB).
IfIndex commands configured for an interface apply to all subinterfaces on that interface.
Examples
In the following example, ifIndex persistence is enabled for interface ethernet 0/1 only:
router(config)# interface ethernet 0/1router(config-if)# snmp ifindex persistrouter(config-if)# exitIn the following example, ifIndex persistence is enabled for all interfaces, then disabled for interface ethernet 0/1 only:
router(config)# snmp-server ifindex persistrouter(config)# interface ethernet 0/1router(config-if)# no snmp ifindex persistrouter(config-if)# exitRelated Commands
snmp-server ifindex persist
To globally enable ifIndex values which will remain constant across reboots for use by SNMP, use the snmp-server ifindex persist command in global configuration mode. To globally disable ifIndex persistence, use the no form of this command in global configuration mode.
snmp-server ifindex persist
no snmp-server ifindex persist
Syntax Description
This command has no arguments or keywords.
Defaults
This command is disabled by default.
Command Modes
Global configuration mode
Command History
12.0(11)S
This command was introduced.
12.1(5)T
This command was integrated into the 12.1 T release train.
Usage Guidelines
Interface Index Persistence means that ifIndex values in the IF-MIB persist across reboots, allowing for consistent indentification of specific interfaces using SNMP.
The snmp-server ifindex persist global configuration command will not override interface-specific configuration. Interface-specific configuration of ifIndex persistence is performed with the [no] snmp ifindex persist and snmp ifindex clear interface configuration commands.
The [no] snmp-server ifindex persist global configuration command enables and disables ifIndex persistence for all interfaces on the routing device using ifDescr and ifIndex entries in the ifIndex table of the IF-MIB.
Examples
In the following example, ifIndex persistence is enabled for all interfaces:
router(config)# snmp-server ifindex persistNote that in this example if ifIndex persistence was previously disabled for a specific interface using the no snmp ifindex persist interface configuration mode command, ifIndex persistence will remain disabled for that interface. The global ifIndex command does not override the interface specific commands.
Related Commands
Glossary
The following definitions are based on RFC 2233, "The Interfaces Group MIB using SMIv2." The terms are values in the Interfaces MIB (IF-MIB).
ifDescr—A description of the interface. Recommended information for this description includes the name of the manufacturer, the product name and the version of the interface hardware/software.
