|  | 7.2 | Monitoring the Network
| 7.2.5 | Remote monitoring (RMON)

Probes gather remote data in RMON. A probe has the same function as a SNMP agent. A probe has RMON capabilities; an agent does not. When working with RMON, as with SNMP, a central management console is the point of data collection. An RMON probe is located on each segment of the network monitored. These probes can be dedicated hosts, resident on a server, or included in a standard networking device such as a router or switch. These probes gather the specified data from each segment and relay it to the management console. Redundant management consoles provide a valuable feature for the network. Redundant management consoles provide two major benefits to network management processes. First is the ability to have more than one network administrator in different physical locations monitor and manage the same network; for example one in New York and one in San Jose. Second is the all-important concept of redundancy. Having two or more management consoles means that if one of the consoles fails, the other console still can be used to monitor and control the network until the first console is repaired. The RMON extension to the SNMP protocol creates new categories of data. These categories add more branches to the MIB database. Each of the major categories will be explained in the following list. 1. The Ethernet Statistics Group Contains statistics gathered for each monitored subnetwork. These statistics include counters (incremental that start from zero) for bytes, packets, errors, and frame size. The other type of data reference is an index table. The table identifies each monitored Ethernet device, allowing counters to be kept for each individual Ethernet device. The Ethernet Statistics Group provides a view of the overall load and health of a subnetwork by measuring different types of errors including CRC, collisions, over and under-sized packets. 2. The History Control Group Contains a data table that will record samples of the counters in the Ethernet Statistics Group over a specified period of time. The default time set up for sampling is every thirty minutes (1800 seconds) and the default table size is fifty entries giving a total of twenty-five hours of continuous monitoring. As the history is created for the specified counter, a new entry is created in the table at each sample interval until the limit of fifty is reached. Then as each new entry is created the oldest entry in the table is deleted. These samples provide a baseline of the network and can be used to compare against the original baseline to resolve problems or to update the baseline as the network changes. 3. The Alarm Group Uses user specified limits that are called thresholds. If the data counters being monitored cross the thresholds, a message or alarm will be sent to the specified people. This process, known as an error trap, can automate many functions of network monitoring. Instead of having a person constantly and directly monitoring the network or waiting for a user to identify a problem with the network, the network process itself can send messages to the network personnel because of a failure or, more importantly, an impending failure. This is an important component of preemptive troubleshooting. 4. The Host Group Contains counters maintained about each host discovered on the subnetwork segment. Some of the counter categories maintained are Packets, Octets, Errors, and Broadcasts. Types of counters associated with each of the previously mentioned items could be, for example, Total packets, Packets received, Packets sent, along with many counters specific to the type of item. 5. The Host TOPN Group Is used to prepare reports about a group of hosts that top a statistical list based on a measured parameter. The best way to describe this group is by example. A report could be generated for the top ten hosts generating broadcasts for a day. Another report might be generated for the most packets transmitted during the day. This category provides an easy way to determine who and what type of data traffic most occupies the selected subnetwork. 6. The Matrix Group Records the data communication between two hosts on a subnetwork. This data is stored in the form of a matrix (a multi-dimensional table). One of the reports that can be generated from this category is which host utilizes a server. Reorganizing the matrix order can create other reports. For example, one report might show all users of a particular server, while another report shows all the servers used by a particular host. 7. The Filter Group Provides a way that a management console can instruct an RMON probe to gather selected packets from a specific interface on a particular subnetwork. This selection is based on the use of two filters, the DATA and the STATUS filter. The data filter is designed to match or not match particular data patterns allowing for the selection of that particular data. The status filter is based on the type of packet looked at. This means, for example, a CRC packet or a Valid packet. These filters can be combined using logical "and" and "or" to create very complicated conditions. The filter group allows the network administrator to selectively look at different types of packets to provide better network analysis and troubleshooting. 8. The Packet Capture Group Allows the administrator to specify a method to use to capture packets that have been selected by the Filter Group. By capturing specified packets the network administrator can look at the exact detail for packets that meet the basic filter. The packet group also specifies the quantity of the individual packet captured and the total number of packets captured. 9. The Event Group Contains events generated by other groups in the MIB database. An example could be that a counter exceeding the threshold for that counter specified in the Alarm Group. This action would generate an event in the Event Group. Based upon this event an action could be generated, such as issuing a warning message to all the people listed in the Alarm Groups parameters or creating a logged entry in the event table. An event is generated for all comparison operations in the RMON MIB extensions. 10. The Token-Ring Group Contains counters specific to token-ring networks. While most of the counters in the RMON extensions are not specific to any type of data-link protocol, the Statistics and History groups are. They are particularly attuned to the Ethernet protocol. The Token-ring Group creates counters necessary to monitor and manage token-ring networks using RMON. It is important to remember that RMON is an extension to the SNMP protocol. Specifically, this means that while RMON enhances the operation and monitoring capabilities of SNMP, SNMP is still required for RMON to operate on a network. As a last point, it is important to mention that there are later revisions of both SNMP and RMON. They are labeled as SNMPv2 and RMON2. This curriculum does not cover all the new capabilities of these versions.
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