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Technical Tip - Connecting a Protocol Analyzer

 

 

How do you connect a protocol analyzer to a "full-up" (no available port) switch or router?
By Barry Nance

Next time you use a protocol analyzer to search for clues to a problem, you might want to take a four- or eight-port hub and extra cable with you.

Connecting a protocol analyzer to a switch to eavesdrop on network traffic typically entails first configuring the switch for port mirroring. By definition, switches send each network message to the specific port the destination device is connected to. Simply plugging a protocol analyzer into a spare switch port doesn’t give you access to the messages flying through the switch.

Configuration steps for enabling port mirroring vary by switch manufacturer and sometimes by model. You may need to bring along a technical manual to help refresh your memory on how to enable port mirroring for the specific switch that’s near the problem you’re trying to solve. And you have to remember to turn off port mirroring when you’re finished.

Accessing a switch’s configuration may be difficult. Perhaps the serial cable for connecting to the switch’s RS-232 port is missing. Perhaps the computers that are physically near the switch are on a different subnet and have no Telnet access to the switch. You may need to phone someone who has both authority over and network access to a switch to get that person to reconfigure the switch before you can begin your protocol analyzer session.

Even finding an available port on a switch is sometimes impossible. You may find that every port has a device already connected to it.

An inexpensive hub and spare cable can quickly and easily solve all these problems. Unlike a switch, a hub echoes network messages across all its ports. Each receiving device is responsible for picking out the messages addressed to it. Each port carries all traffic and is not device- or address-specific.

Here’s an effective technique for using a hub to gain easy access to the connection between, say, a router and a switch.

First, notify the appropriate people that you’re going to momentarily interrupt network traffic to the switch. Then, when you’re at the switch, connect a spare cable to the hub. Quickly and smoothly unplug the cable linking the router to the switch and use that cable to connect the router to the hub. Then connect the spare cable already plugged into the hub to the switch.

Voila! All traffic to and from the router will pass through the hub, to which you now connect your protocol analyzer and begin your investigations.




Copyright 2012 Network Testing Labs


  
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