need to change remote subnets

Unanswered Question
Feb 24th, 2010

I need to change remote subnets from 25 to 25 in a remote site.  Right now the site has a 2800 router connected to a 3560 switch, both running IOS.  I'd like to split the 24 bit subnet connecting the switch to the router into 2 25 bit subnets - one the upper half of the existing 24 bit subnet, and the other the lower half, each on a different vlan.  I only have a modem connected into the router, and I'd like to do make the change without losing connectivity to the switch.  The router and switch are not set up for dot1q trunking.  So what I'm doing is creating an additional vlan on the switch and spliting the existing 24 bit subnet accross the new vlan and existing vlan.  Without a modem in both the router and switch, I'd like to do this without losing connectivity, so I don't end up with the interface on the swith having no IP address.


so in the above diagram, the switch's interface to the router would become (ni a vlan with that IP address) and the router's interface to the switch would become  Then a new vlan with the IP address would be created, and a static route to that vlan put into the router.  BGP would take care of rest.  I'd consider also trunking between then two, but again I'm not sure what state the switch's configuration will be left in if connectivity is lost.

Any suggestions would be very helpfull.


I have this problem too.
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Leo Laohoo Wed, 02/24/2010 - 16:46

Save the configs and copy them to your PC using the "copy flash:startup-config tftp:" commands.  Use Wordpad (do NOT use Notepad) and make the necessary changes to the config and push the configs back up (copy tftp: flash:running-config).  If you don't like the result, you can invoke the copy start runn.


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