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Will there be any problem with two routers with same E0 ip address

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Oct 8, 2000
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US
Our business is moving from 56k lines to T1 lines. Currently, each remote office has a 56k connection (thru a bay router) back to our main office. I am installing a new Cisco 1720 router at each remote office with a T1 card with a built in CSU/DSU.

For a month, I want to have both lines (56K & T1) active from the home office to all remote offices. This way there is redundency in case of a line problem with the T1 or 56k.

The 56k line is running RIP. The T1 is running OSPF.
We will eventually stay with OSPF. All the PC's have a default gateway of 192.168.120.1

The Bay 56K router at the remote office has this config.
E0 of 192.168.120.1
S0 of 192.168.20.2 (home office router S0 is 192.168.20.1)

The Cisco T1 router at the remote office has this config.
E0 of 192.168.120.1
S0 of 192.168.175.2 (home office router S0 is 192.168.175.1)

Note that the E0 on both routers has the same IP address.
Also, the 56K line is connected to a cisco 3640 at the home office and the T1 line is connected to a 7200 router at the home office. Both the 3640 and the 7200 routers ethernet interfaces are connected to the same Cisco 5500 switch at the home office.

My take on this is that there will be no problem having the same IP address on the E0 of the remote office routers and that the least cost path will always be used. (T1 on OSPF). And if the T1 line goes down, that data will just start moving across the 56K line automotically. Is this theory correct or am I missing something??
Thanks.
 
OSPF sounds like a good idea , however I'm worried about the LAN. Duplicate ip's on
the ethernet seems like it would create a conflict to your pc's .( possibly creating a loop ) ! I would try another approach , first if your worried about the T1's why not just add a 56k wic card to the new 1720 router and run EIGRP for load balancing . That way you only have ONE ethernet and Two WANS . Second go ahead and just remove
the 56k lines and just use the t1's . When your telco company is out to install the T1's
verify right away that the span is working , Thus eliminating future issues on the circuit. Sorry I could not be of more help at this time . If and when you complete this I
would like to know the outcome and of any troubles that may have occured .
Good Luck !!!! Jeter@LasVegas.com
 
We don't have the budget to add a 56k WIC card to the 1720. So this is not an option. And since the remote locations are a bit of a travel, I don't want to have to go out there to reconnect the 56k lines in the event the T1 goes down. I only plan to use this dual method for about a month, just until I am positive there are no T1 problems. It's better to be safe than sorry (these are doctors offices).

Currently, I have this dual method setup and it seems to be working just fine. The PC's are picking the least cost path to send data (ospf on T1). And when I close the T1 E0 interface, the data automatically starts moving across the 56k line.

I just don't know the long term issues. Anyone else have any ideas on what could go wrong.
Thanks
 
The duplicate IP's are defenitly a problem. Give the T1 connection the *.*.*.1 address making it the main gateway.
It is not clear if you want to permenantly keep the 56k Line. Or you want to plan a perfect transition. In any case give the 56k router *.*.*.2 address. Then assign both address as gateway address to the network clients. If the T1 goes down the clients will resort to the next gateway address in the list. Also, you may give the T1 router a gateway of last resort pointing to the 56 K router.

If you plan to keep the 56K line as a backup the only logical thing to do is get another wan card.

 
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