using cisco 2503 as proxy server

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I have a situation a network which gets its internet connectivity from
ethernet cable and a backup of ISDN connection is required. My internal
pC are connected to a network and my service provider is
giving me a internet connectivity through a ethernet connection with ip
address of and the backup connection is isdn dialup with
dialup no. 172222 and username/password combination.I would like to run
my pc's/host to connect to internet and use msn mailing and skype and
in case of non availability of data connection through internet it
should dial isdn to connect the internet. I think it is possible with a
2503 with configuring my ethernet port with two ip addresses one of and other of as sub interface, connected to the
router with a cable from switch which will have both the 10.x.x.x isp
cable and 192.x.x.x internal network connected. My isdn will be
connected to the BRI port of the router. Kindly help me as this could
cost me my job and with one router in hand i cannot experiment on the
network. i would really appreciate if someone could give me the
configuration woith detailed info. Many thanks in advance and would be
greatful for life.

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