Accton Technology ES4626 - Manual (Page 70)

70
SNMP protocol provides a simple way of exchange network management
information between two points in the network. SNMP employs a polling mechanism of
message query, and transmits messages through UDP (a connectionless transport layer
protocol). Therefore it is well supported by the existing computer networks.
SNMP protocol employs a station-agent mode. There are two parts in this structure:
NMS (Network Management Station) and Agent. NMS is the workstation on which SNMP
client program is running. It is the core on the SNMP network management. Agent is the
server software runs on the devices which need to be managed. NMS manages all the
managed objects through Agents. The switch supports Agent function.
The communication between NMS and Agent functions in Client/Server mode by
exchanging standard messages. NMS sends request and the Agent responds. There are
seven types of SNMP message:
z Get-Request
z Get-Response
z Get-Next-Request
z Get-Bulk-Request
z Set-Request
z Trap
z Inform-Request
NMS sends queries to the Agent with Get-Request, Get-Next-Request,
Get-Bulk-Request and Set-Request messages; and the Agent, upon receiving the
requests, replies with Get-Response message. On some special situations, like network
device ports are on Up/Down status or the network topology changes, Agents can send
Trap messages to NMS to inform the abnormal events. Besides, NMS can also be set to
alert to some abnormal events by enabling RMON function. When alert events are
triggered, Agents will send Trap messages or log the event according to the settings.
Inform-Request is mainly used for inter-NMS communication in the layered network
management.
USM ensures the transfer security by well-designed encryption and authentication.
USM encrypts the messages according to the user typed password. This mechanism
ensures that the messages can’t be viewed on transmission. And USM authentication
ensures that the messages can’t be changed on transmission. USM employs DES-CBC
cryptography. And HMAC-MD5 and HMAC-SHA are used for authentication.
VACM is used to classify the users’ access permission. It puts the users with the
same access permission in the same group. Users can’t conduct the operation which is
not authorized.
Introduction to MIB
The network management information accessed by NMS is well defined and