Connect 2 routers in parallel

Posted on 2004-11-02
Last Modified: 2008-03-10
Hi All,

I have a need to connect 2 business routers to one T1 circuit.  The current router, which provides internet access to my network, is a Cisco 1721 series.  We are migrating to an MCI VOIP solution, and an MCI tech needs to ping the new router, a Siemens SE5940.  Needless to say the tech is very hard to reach, and all I've been able to squeeze out of him is that the routers "need to be in parallel."  I'm not even sure what the IP address of the new router  is, but I don't think I need to know it.  All I know is that somehow I need to provide ping access to the new Siemens router, while keeping access to the Cisco router.  Can this be done with a hub?  I know that a switch will not work.  


Question by:bensbenchcs
    LVL 12

    Accepted Solution

    You state that you are connecting to a T1, if the T1 is terminated directly into the Cisco router then using a hub or switch will not work. If the T1 is terminated into carrier equipmnet, and then the Cisco is connected via the Ehternet interface, then you might be able to use a hub or even a switch, as long as the second router connecting can obtain an IP address or be assigned an IP address that will alolow connection to the gateway of the Cisco router.

    If the T1 terminates directly into the Cisco, then you need to do some work.

    Solution A

    Connect second router to second ethernet interface on Cisco router if availiable, then assign external IP address to second router. Allow all request recieved at Cisco Router for IP address of second ruoter to go over the second ehternet interface.

    Solution B

    Connect a hub/switch betwen Cisco router Ehernet port and network, connect second router to hub/switch, assign  IP address and allow ping request/telnet traffic from outside to the IP address of the second router.

    Would need your config to be more exact, also network layout

    Keep me posted

    Author Comment

    Hi Joel,

    I don't have access to either of the routers-the office in question has an MCI-provided T1.  I cannot configure the cisco, nor can I configure the task is just to connect the routers in parallel to the T1 so that all normal internet traffic goes through the cisco (basically cisco remains untouched) and MCI can ping the new Siemens router so we can get rolling with our new VOIP solution.  I'm guessing the external IP for the Siemens has already been assigned, and all I need to do is connect it in parallel with the cisco.  It sounds like, based on your previous comment, that I should be able to connect a hub/switch between the cisco and our office firewall and connect the Siemens to the hub/switch.  To Summarize,

           /        \
     Siemens    Firewall
                       Office Network

    This doesn't look parallel to me, but would an outside party be able to ping the Siemens if it has an already pre-configured IP address based on our network? MCI should have access to both the Cisco and Siemens configuration.  

    Thanks for your time,

    LVL 12

    Expert Comment

    If the Firewall can be pinged, and the Siemens box can obtain an IP address from the Cisco Router, then you should be able to ping the Siemens router.

    Good part of this depends on the Cisco router configuration, also are using private or public address for your office network? If you use private and using a DHCP server, the Siemens box can not obtain a IP address from the local network, the Firewal should block those type of requests. So then the Siemens box will need to obtain the IP address from the Cisco router, or if the Cisco router is preconfigured for a certian block of Public IP addresses then you would need to assign one to the Siemens box manually.

    Hope I have communicated this clearly to you. Let me know.

    Kindest regards
    LVL 12

    Expert Comment

    How are things coming along?

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