SNMP management information travels the same network path as your data. It uses the same WAN and LAN routers, hubs, communications links, and DSU/CSUs. And there lies the problem. While the network is operating normally (or hovering near the normalcy), SNMP packets flow between the managed devices and the management workstation or Remote Monitor (RMON) with no problem. Its TRAPs, SETs, and GETs all flow with the same priority as regular traffic on the LAN/WAN and provide management information to the workstation or commands to the controlled devices. BUT, when the network goes down or is severely disrupted, SNMP traffic has no way to get between the managed device and the management workstation. Telnet is usually used in conjunction with an SNMP workstation. Telnet packets are also unable to move between the management workstation and managed devices during those network disruptions.
Whats the solution? Out-of-Band network management. With out-of-band management, a second path is available to the managed devices that does not depend on the LAN/WAN. It usually consists of an RS-232 access switch connected to the management port of each controlled device. For remote access, connect a modem and dial-in telephone line. This provides a direct route to the management port of each device that can be used for reconfiguration, troubleshooting, and rebooting. This route is not dependent upon telnet or SNMP packets moving through the LAN/WAN system. Every network device that supports SNMP also contains a RS-232 management port. Although it doesnt provide the fancy GUI interface of most SNMP workstations, this method provides the native interface for each device being controlled. For example, a Cisco router presents its IOS console prompt; a Netware server has its Rconsole colon prompt available. The access switch is transparent when connected to any device.
Do the two methods (in-band SNMP and Out-of-Band) compete with each other? Of course not, they compliment each other. System cost increases only slightly when adding out-of-band management; functionality increases an order of magnitude. Use the SNMP GUI where it fits best... for normal network monitoring and metrics, alarm reports, and data reduction. Then, when problems surface, run terminal emulation on the workstation and connect to the remote access switch for direct control of remote equipment. Since there is direct access to the management port, troubleshooting doesnt have the added complexity of a network being between the device and the technician. And when the network is down, the same connectivity is still there... a direct pipe between the technician and the device causing the problems.
How does terminal server access compare? Most devices can be connected to the network through a terminal server and their management port. Is that the same as an access switch device? NO! The terminal server places the data path in-band. If your LAN/WAN goes down, there is no path to the managed device. The key is that an access switch provides an independent, out-of-band path to the controlled devices.
Is SNMP management still needed with an access switch? When managing a mid-size to large network, SNMP cant be beat for the day-to-day management functions and metrics. It provides management reports concerning the state of the network and helps in planning growth paths. SNMP is also great for alarming faults. So, it should be used in conjunction with a good out-of- band access method.
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