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Message ID: 2200     Entry time: Sun May 27 16:10:27 2018
Author: awade 
Type: Computing 
Category: Computing 
Subject: Accessing 3com switch from screen on mac 

Somehow I lost access to the 3com switch in the PSL lab.  The default config IP was lost and from there we have been unable to edit its settings.

The 3com model 2920-SFP switches don't have a reset button.  They can only be reset from the serial port.  Larry Wallace lent me a USB serial to RJ45 cable but I have been unable to obtain a connection through a terminal PuTTY session until now.

The instructions for resetting the router can be found HERE

I found that serial connections through MacOs terminal 'screen' utility just returned random characters. It turns out that the problems with the USB serial connection were the baud rate. In this case it must be set to 38400

Connecting to 3com routers with USB converter

To launch a serial USB session with a converter device. Run

> ls /dev/*usb*

then plug in the USB converter device (USB to serial DB9 + DB9 to RJ45) and run the above command again to see what appears when the device is initialized. You should see something like /dev/cu.usbserial-xyz appear, where cu.usbserial-xyz is serial converter you want to connect to.

Then launch screen with

> screen -L /dev/cu.usbserial-A4007BdM 38400 -L

Power cycle the 3com router and it should return a bunch of sensible startup dialog like


    *                                                                      *
    *              3COM 2920-SFP Plus BOOTROM, Version 113                 *
    *                                                                      *
    Creation Date       : Jun  1 2009
    CPU L1 Cache        : 32KB
    CPU Clock Speed     : 333MHz
    Memory Size         : 128MB
    Flash Size          : 128MB
    CPLD Version        : 002
    PCB Version         : Ver.B
    Mac Address         : 002473850035
Press Ctrl-B to enter Extended Boot menu...0


As instructed hit Control-b (Not command-b) then follow the instructions in the LINK above. Note that where they say "<blank>" for password they actually mean enter nothing.

From there you can configure IP addresses etc from command line.  However, it is probably just easier to let the top level router DHCP allocate an IP address and then do it directly from the browser.


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