Document Number: OSUP1025
Product: Observer 4.x or greater
Date: 11-15-99
Title: Observer’s Use of Exploratory Packets - Observer broadcating LLC packets

Observer’s Use of Exploratory Packets - Observer broadcating LLC packets

Summary
This document deals with Observer’s use of LLC packets to monitor a LAN’s efficiency and test for collisions. While these packets use a very small amount of network bandwidth, some users find any routine packet generation undesirable, and may wish to choose when such capabilities are enabled and disabled.

Keywords:
Observer, Packets, LLC, exploratory

Problem
Some users have noticed that Observer, under certain circumstances, generates several hundred LLC-type packets on the network every minute.

Observer uses such packets for two purposes. When the Efficiency History Mode is active, it regularly sends a very small burst of LLC packets out to test the network’s efficiency, similar to the way that radar sends out regular beeps to see how long it takes for the beeps to bounce back. Vitals Signs Mode can also send out a larger—but still very small—number of such exploratory packets to test for collisions.

While this is a tiny amount of the traffic on a network, some users prefer to have Observer be completely passive on the network.

Solution
There are two modes of Observer which produce such exploratory LLC packets. Either or both can easily and independently be turned on and off by the user.

1) Efficiency History
The first such mode is Efficiency History, which sends LLC packets addressed to 00:04:00:00:FF. This can be started and stopped at any time.

The Efficiency History test was designed to provide an ongoing snapshot of your LAN’s current efficiency.

When the Efficiency History mode is active, a test is run every 10 seconds. This test consists of Observer bursting 70 packets (for Ethernet and Fast Ethernet), or 30 (for Token Ring) onto the LAN. On a 10-megabit network, this will use an average of, at most, 0.28% of the network’s bandwidth; on a 100-megabit network, it will use at most 0.028% of the bandwidth.

While these amounts are negligible, and most users will prefer to have Observer maintain an ongoing picture of the network’s efficiency, some users will prefer to turn such use off and on.

To disable the sending of these packets, simply turn off this mode by closing the Efficiency History Window. Observer will immediately cease transmitting Efficiency History exploratory LLC packets. To enable monitoring of Efficiency History, click “Modes”, then “Efficiency History” on the main menu. Efficiency History will begin transmitting exploratory packets.

2) Vital Signs
The second possible source of such exploratory LLC packets is the Vital Signs Mode. In Vital Signs, Observer may be running the collision test. When the collision test is option is on, Observer will burst 100 exploratory LLC packets per second, addressed to 00:00:FF:FF:FF,  and listen for packet collisions.

This will use a larger percentage of bandwidth than Efficiency History testing, but still a small amount. On a 10-megabit network, collision testing (when active) will use 4% of the network’s bandwidth; on a 100-megabit network, it will use 0.4%.

Collision testing is generally run when a collision problem is suspected, although it can be run routinely, at the user’s option.

As with Efficiency History, the collision test in Vital Signs can easily be turned off when not needed. To disable collision testing, click on the Vital Signs window, then click “Mode Menu”, “Setup” and then uncheck “Run Collision Test.”.

To enable collision testing, click on the Vital Signs window, then click “Mode Menu”, “Setup” and then check “Run Collision Test”.

Conclusion
Two Observer modes—Efficiency History and Vital Signs—can generate bursts of exploratory LLC packets that some users may find undesirable. While most users will wish to routinely leave the Efficiency History testing on and Vital Signs’ collision testing off, either of the tools can easily and independently be turned off and on by the user.

For further information, please contact your Network Instruments sales representative or a support representative at (800) 526-7919, or outside the US at (952)-939-9899.