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Single Domain - Multiple sites with default gateway issues

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  • Single Domain - Multiple sites with default gateway issues

    First off I want to apologize, i read the rules and tried to search but I don't know what I don't know so I couldn't figure out what to search for.

    Here is the scenario:

    i have three locations, one location with a network of 10.198.148.0 one with 10.198.165.0 and one with 10.198.124.0 which I have subnetted all of them to /28.

    Router 1 I set G0/0 to 10.198.148.122 255.255.255.128 and G0/1 to the external ip given to me by another group. Call this network A

    Router 2 I set G0/0 to 10.198.165.126 255.255.255.128 and G0/1 to the external ip given to me by another group. Call this network B

    I have not set up the 3rd location yet but it is not completely irrelavent here.

    A switch on Network A has Vlan2 with an ip of 10.198.148.101 and several servers hanging off it.

    A switch on Network B has Vlan 2 with an ip of 10.198.165.101 and several servers hanging off of it.

    My real question is how do these routers and networks know what the Default Gateway is? They will all communicate internally within their own respective networks just fine but will only communicate between the 2 networks when i set the router port G0/0 = to the default gateway. I know this is wrong, i just do not know enough about it to correct it.

    What am I missing in the router configuration steps that it doesn't know how to forward to the port.


  • #2
    Re: Single Domain - Multiple sites with default gateway issues

    You need a layer 3 device (router or L3 Switch) to route between different subnets.
    When a router has an interface on the same ip subnet as your hosts then it will create a route in its local routing table for that network. On a cisco device this will show up as a "connected route". In order for a router to reach a remote subnet it needs a route to that network. This can be a static route, a default route or learned by a dynamic routing protocol. The remote subnet will also need a route back to the originating router who sent the packet.

    Routers use the routing table for packet forwarding. Hosts on your network use the default gateway to send packets to destinations not on its local subnet. The default gateway is usually handed out as part of the dhcp lease.



    I cant see your configs and/or a diagram, so I dont know what your configuration looks like.
    Last edited by auglan; 5th June 2012, 22:43.
    CCNA, CCNA-Security, CCNP
    CCIE Security (In Progress)

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    • #3
      Re: Single Domain - Multiple sites with default gateway issues

      Here is a config for a router and a switch. I am pretty sure once one is configured correctly i will have my answer for all of them.
      Attached Files

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      • #4
        Re: Single Domain - Multiple sites with default gateway issues

        If your going to keep the switchstack as layer 2 and you need to encapsulate multiple vlans then you would need a trunk to the router and then sub-interfaces on the router for each subnet. Is vlan 2 the only vlan you need on that stack? From looking briefly at the config, your vlan 2 hosts should be able to reach their gateway which I assume is 10.198.165.126. From there that router will need routing entries for any other remote network.

        ip route 10.198.165.0 255.255.255.128 10.198.164.32 - This tells the router to get to 10.198.165.0 go to the next hop of 10.198.164.32. This route makes no sense as the 10.198.165.0/25 network is local to the router (gi0/0 connected to the switch) Since this is the only route and there is no default route set than I can assume that vlan 2 hosts can't get to anything not locally connected to this router.


        A better option would be to enable routing on the switch stack since they are 3750's and either use access ports to the routers or run routed ports directly between them. You would need to create an SVI (Switched Virtual Interface for any vlans you need routed) You could then use static routing or run eigrp (since I see it enabled on the router) with the router. This config is easier because if you need to add a new subnet all you need to do on the switch is create the vlan and corresponding SVI, then configure your hosts to use that SVI as their default gateway.



        interface Vlan2
        ip address 10.198.165.101 255.255.255.128

        This interface is used only for remote management and so is the ip default-gateway command on the switch. Once your turn on routing then you can remove the ip default-gateway command.

        ip routing
        Last edited by auglan; 6th June 2012, 11:52.
        CCNA, CCNA-Security, CCNP
        CCIE Security (In Progress)

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        • #5
          Re: Single Domain - Multiple sites with default gateway issues

          Thanks for the advice, I think i will enable routing on the stack and go that route. I'll let you know how it works out.

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