Routes between 2 branch offices and main office

I have one main office and two branch offices. The main office has 1 main router and 2 routers that are there to talk to the satelite offices. These two routers also connect to the main router. Each office is connected to the main office via a T1 connection. Each office has a CSU/DCU unit for the T1 line and a router. All routers are Cisco except the main one in the office. I am having trouble getting the branch offices to talk to the main office. I think the problem lies in the routing of the routers. I can ping both ethernet and serial links on of the router but only the serial link on another. Can someone explain what the correct routes should be from each office to the main office and what routes should be setup on the routers in the main office and the main router. Each router has ethernet0 and serial0 setup.
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but if the main office has been up for ages and others are talking to it
then better not add this ip route before checking whats the routing running on it
do a sh ip route (if it is cisco) and sh ip protocols brief
if it is running some routing protocol then probably we need to do the same on the others
if it is just default then no problem

I would like to know if you are running any routing protocol in between ?
Supposingly you can ping the serial link because it is a point to point connection but your main router should have knowledge about the other networks in order to reach them

try to do a sh ip route "ethernet of the other side" and see if it is there or how is it known
Try also to do a trace route sourced by your ethernet and see the outcome
vmamedovAuthor Commented:
Hi logic2,
What i need help with is adding routes to the routers. Lets pretend that these routers dont have any static routes right now and I need to add them. For example I can remember one thats x.x.x.x and so on, or something like ethernet0 subnet mask to serial link
well then if you just need to perform static routing you can add this

ip route "ethernet network ip of the other other router" "subnet mask" serialx
ip route serialx  <<< this should be enough i guess as this is a default route routing traffic our to the WAN link

the above route should be put on both routers (both ends of the link)

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