Network Loops / Bridging

With the release of Windows XP, Microsoft added the capability to "bridge" or effectively link one network to another through a PC. This is quite useful in a home environment if one wanted to set up a wireless network without buying a dedicated wireless access point. But it causes problems at a LAN Party because users aren't aware of how bridging works and sometimes just bridge everything!

Or perhaps a user went through the wizard and picked the wrong settings, and never noticed a problem because he or she was never connected to both a wireless and wired network at the same time at home. But when the wired and wireless networks are bridged at a LAN Party, normal network "chatter" will be continuously looped. (The PC sends a broadcast, it hits the bridge, goes out again over the wireless and is linked back to the core via the wired network.)

Fortunately, bridges are relatively easy to detect by examining the core switch's MAC Address Lookup tables, like so:

hp2824# show mac-address

Status and Counters - Port Address Table

MAC Address Located on Port
------------- ---------------
 001111-46ccdc 1
 001279-c6188b 2
 001111-46ccdc 2
[snip]

In the example above, port 1 has a wireless access point connected and port 2 is connected to a floor switch. You can see the same MAC address appearing on both ports 1 and 2. This normally should not happen and indicates a probable bridge in use. Begin problem mitigation by making an announcement that all wireless users should disconnect from the wired network until they can verify that their system is not bridging.

Tags: How To, LAN,

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