The architecture for describing Internet Management Frameworks contained in RFC 3411 describes the SNMP engine as composed of the following components:

• Dispatcher
• Message Processing Subsystem
• Security Subsystem
• Access Control Subsystem

Applications make use of the services of these subsystems. It is important to understand the SNMP architecture and the terminology of the architecture to understand where the Security Model fits into the architecture and interacts with the other subsystems within the architecture. The information is contained in RFC 3411 and your are encouraged to review this RFC to obtain an understanding of the SNMP architecture and subsystem interactions.

In the AES and 3-DES Encryption Support for SNMP Version 3 feature the Cipher Block Chaining/Data Encryption Standard (CBC-DES) is the privacy protocol. Originally only DES was supported (as per RFC 3414). This feature adds support for AES-128 (as per RFC 3826) and AES-192, AES-256 and 3-DES (as per CISCO-SNMP-USM-OIDS-MIB).

• AES encryption uses the Cipher Feedback (CFB) mode with encryption key sizes of 128, 192, or 256 bits.
• 3DES encryption uses the 168-bit key size for encryption.

The AES Cipher Algorithm in the SNMP User-based Security Model draft describes the use of AES with 128-bit key size. However, the other options are also implemented with the extension to use the USM. There is currently no standard for generating localized keys for 192- or 256-bit size keys for AES or for 168-bit size key for 3-DES. There is no authentication protocol available with longer keys.

The AES and 3-DES Encryption Support for SNMP Version 3AES and 3-DES Encryption Support for SNMP Version 3 feature supports the selection of privacy protocols through the CLI and the Management Information Base (MIB). A new standard MIB, SNMP-USM-AES-MIB, provides support for the 128-bit key in AES. The extended options of AES with 192- or 256-bit keys and 3-DES are supported as extensions to the SNMP-USM-MIB, in the Cisco-specific MIB, CISCO-SNMP-USM-OIDS-MIB.

1.    enable

2.    configure terminal

3.    snmp-server user username group-name [remote host [udp-port port]] {v1 | v2c | v3 [encrypted] [auth {md5 | sha} auth-password]} [priv {des | 3des | aes {128 | 192 |256}} privpassword] [access [ipv6 nacl] {acl-number | acl-name}]

To display information about the configured characteristics of Simple Network Management Protocol (SNMP) users, use the show snmp user command in privileged EXEC mode.

Note	The show snmp user command displays all the users configured on the router. However, unlike other SNMP configurations, the snmp-server user command will not appear on the "show running" output.

1.    enable

2.    show snmp user [username

Router# show snmp user
 abcd
User name: abcd 
Engine ID: 00000009020000000C025808 
storage-type: nonvolatile       active access-list: 10
Rowstatus: active 
Authentication Protocol: MD5
Privacy protocol: 3DES 
Group name: VacmGroupName 
Group name: VacmGroupName