T1 to T1 with shared Internet

Unanswered Question
Feb 25th, 2010
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Hello all!

There is a T1 already in place at the main office and they are running in the IP range of (the default gateway is  They have a Cisco router that they want us to configure with a T1 CSU/DSU to a branch office that is getting a new T1 installed.  They are currently using DSL at the branch office and will be getting that disconnected soon.  They want the internet from the main office to be used at the branch, which is going to be on the IP  Here is what my current intereface looks like for the router at the branch office...

interface GigabitEthernet0/0
ip address
duplex auto
speed auto
interface Serial0/0/0
ip unnumbered GigabitEthernet0/0
encapsulation ppp
no clock rate 2000000
service-module t1 clock source internal
service-module t1 timeslots 1-24

What else needs to be added so that the branch office knows where to forward it's internet traffic?  And will the main office and the branch office be able to share files?  Thanks again!

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Jon Marshall Thu, 02/25/2010 - 12:11
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Basically you just need to take care of routing and NAT.

1) routing - you could run a dynamic routing protocol between your 2 sites or you could just use statics. If statics -

main office router -

ip route

this will tell your main office router how to get to branch office network

branch office router -

ip route  <--- note i'm guessing what this IP is as you haven't included the main office config

the above will send all traffic to main office ie. both traffic to main office network and the internet

2) If you have NAT setup for internet use for your users in main office then you  may need to add to your NAT config. It's difficult to say without knowing how you have setup NAT at the moment.

With this setup you should be able to share files between sites and have branch users get to the internet.



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